(CNN) A stampede during one of the last rituals of the Hajj -- the annual Islamic pilgrimage to Mecca -- has killed more than 700 people and injured nearly 900 others in Saudi Arabia. 
 
 The stampede occurred Thursday morning during the ritual known as "stoning the devil" in a tent city in Mina, about two miles from the holy site in Mecca, Islam's holiest city. 
 
 Footage obtained by CNN Arabic shows a disturbing scene. Bodies piled upon bodies, a few moving, but most appearing lifeless. Workers in hard hats and reflective vests can be seen pulling dead bodies away to get to those who are still alive. 
 
 The video captures the cacophony of shouts amid the chaos. 
 
 Ethar El-Katatney, a pilgrim who was near the stampede site about five hours after the surge happened, said she walked past ambulances carrying bodies of victims. She said she saw numerous police officers and medical personnel in the area. 
 
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 "I saw the ambulances, I saw bodies. ... At least 20, 30 ambulances passed me by," she told CNN by phone. 
 
 The incident is the deadliest disaster at Mina since 1990, when 1,426 people died. 
 
 Civil defense authorities said the latest death toll is 717, with 863 people injured, but the numbers have been climbing steadily. Officials deployed 4,000 workers, along with 220 ambulances and other vehicles, to Mina in response to the disaster. 
 
 In the ritual, crowds of pilgrims throw stones at three pillars in a re-enactment of when the Prophet Abraham stoned the devil and rejected his temptations, according to Muslim traditions. 
 
 Crown Prince Mohammad bin Nayef bin Abdulaziz held an emergency meeting to discuss the stampede, according to the Saudi Press Agency. 
 
 فيديو: خطة عاجلة تطبقها مستشفيات المشاعر المقدسة لفرز الحالات المصابة بتدافع منى #الإخبارية #تدافع_مشعر_مني pic.twitter.com/nnaDNUrIO9 — قناة الإخبارية (@alekhbariyatv) September 24, 2015 
 
 He ordered an inquiry. 
 
 Saudi King Salman bin Abdulaziz Al Saud later ordered a review of the country's plans during Hajj. 
 
 "Regardless of the investigation results, the improvement of the methods and mechanisms of the Hajj season will not stop. We have instructed the concerned entities to re-evaluate the current policy and the distribution of responsibilities," he said. 
 
 In Thursday's stampede, pilgrims were walking toward the largest of the pillars when there was a sudden surge in the crowd about 9 a.m., causing a large number of people to fall, the Saudi Press Agency said, citing civil defense officials. 
 
 Information on what led to the surge wasn't immediately available. 
 
 Photos: Hundreds killed in Hajj stampede Hide Caption 1 of 14 Photos: Hundreds killed in Hajj stampede Saudi emergency personnel surround bodies of Hajj pilgrims at the site of a stampede Thursday, September 24, in Mecca, Islam's holiest city. Thursday morning's stampede killed hundreds during one of the last rituals of the Hajj, the annual Islamic pilgrimage Hide Caption 2 of 14 Photos: Hundreds killed in Hajj stampede Saudi emergency personnel carry a body. Hide Caption 3 of 14 Photos: Hundreds killed in Hajj stampede People stand near the bodies of victims at the site of the stampede. Hide Caption 4 of 14 Photos: Hundreds killed in Hajj stampede Bodies lie covered in sheets. The death toll has been climbing steadily. Hide Caption 5 of 14 Photos: Hundreds killed in Hajj stampede People stand near bodies covered in sheets. Hide Caption 6 of 14 Photos: Hundreds killed in Hajj stampede Muslim pilgrims and rescuers gather around the victims. Hide Caption 7 of 14 Photos: Hundreds killed in Hajj stampede Bodies lie in the street as emergency personnel work. Hide Caption 8 of 14 Photos: Hundreds killed in Hajj stampede Saudi emergency personnel load a wounded pilgrim into an ambulance. Hide Caption 9 of 14 Photos: Hundreds killed in Hajj stampede Emergency personnel carry an injured person. Hide Caption 10 of 14 Photos: Hundreds killed in Hajj stampede Saudi ambulances arrive on the scene. Hide Caption 11 of 14 Photos: Hundreds killed in Hajj stampede A man searches for a relative injured in the stampede. Hide Caption 12 of 14 Photos: Hundreds killed in Hajj stampede Saudi medics tend to an injured pilgrim. Hide Caption 13 of 14 Photos: Hundreds killed in Hajj stampede Saudi emergency personnel stand near bodies of Hajj pilgrims. Hide Caption 14 of 14 
 
 A risky pilgrimage 
 
 The ceremony was the scene of stampedes and hundreds of deaths in the 1980s and 1990s as pilgrims passed a crowded bottleneck area leading to the small pillars on the ground. 
 
 In 2006, a stampede there killed at least 363 people. 
 
 After that, the Saudi government erected three massive pillars and completed a $1.2 billion, five-story bridge nearby where pilgrims can toss stones. It was meant to be a roomier atmosphere and a more efficient way to accommodate the faithful. 
 
 Speaking at St. Patrick's Cathedral in New York, Pope Francis offered words of comfort during evening prayers. 
 
 "I have two feelings for my Islamic brothers. First my greeting for being today the day of the sacrifice. I would have liked that my greetings would have been warmer," he said. 
 
 "A second feeling is my closeness, my closeness with the tragedy that its people have suffered today in Mecca. In this moment of prayers, I join and we join them." 
 
 The ritual 
 
 The stoning ritual is done over at least two days. Pilgrims stone the three pillars at Mina -- believed to be where the devil was stoned when he tried to dissuade Abraham from obeying God's orders to slaughter his son. According to tradition, the event was a test from God, who gave Abraham a ram to slaughter instead. 
 
 Thursday was the third day of the Hajj. 
 
 On September 11, just days before this year's Hajj started, a construction crane crashed through the roof of another Hajj destination, the Grand Mosque in Mecca, killing 107 people. At least 238 others suffered injuries when a powerful storm toppled the crane. 
 
 Losing one's life during the Hajj season is considered by many devout Muslims as an entry to heaven. 
 
 A spiritual climax 
 
 More than 2 million Muslims from around the world are attending the annual Hajj pilgrimage this year. 
 
 Photos: The Hajj: Muslims gather for annual pilgrimage Photos: The Hajj: Muslims gather for annual pilgrimage Muslim pilgrims make their way to cast stones at a pillar in the "stoning the devil" ritual Thursday, September 24, in Mina near Mecca, Saudi Arabia. At some point during this last rite of the annual Hajj, a stampede occurred, killing and injuring hundreds . More than 2 million Muslims have been making the pilgrimage to the holy city of Mecca. The Hajj is one of the most celebrated events in the Islamic calendar. Hide Caption 1 of 8 Photos: The Hajj: Muslims gather for annual pilgrimage A pilgrim reads the Quran on a rocky hill called the Mountain of Mercy, on the plain of Arafat, near Mecca on Wednesday, September 23. Hide Caption 2 of 8 Photos: The Hajj: Muslims gather for annual pilgrimage Muslim pilgrims pray at Namira Mosque on September 23 on Mount Arafat near Mecca. Hide Caption 3 of 8 Photos: The Hajj: Muslims gather for annual pilgrimage Pilgrims join in one of the Hajj rituals early September 23 on Mount Arafat. For most Muslims, the pilgrimage is the spiritual climax of their lives, with many saving for decades to be able to make the journey to Mecca. Hide Caption 4 of 8 Photos: The Hajj: Muslims gather for annual pilgrimage Muslim pilgrims pray on the Mountain of Mercy, on the plain of Arafat. Islam requires every Muslim who is physically and financially able to make the journey to Mecca at least once in his or her lifetime. Hide Caption 5 of 8 Photos: The Hajj: Muslims gather for annual pilgrimage Muslim pilgrims pray on the Mountain of Mercy on Tuesday, September 22. Hide Caption 6 of 8 Photos: The Hajj: Muslims gather for annual pilgrimage Muslim pilgrims touch Islam's holiest shrine, the Kaaba , at the Grand Mosque in Mecca on Monday, September 21. Performing the Hajj is one of the five pillars of Islam. Hide Caption 7 of 8 Photos: The Hajj: Muslims gather for annual pilgrimage Muslim pilgrims circle counterclockwise the Kaaba at the Grand Mosque in Mecca on Monday. The pilgrimage, conducted over five days, includes detailed rituals including wearing a special garment that symbolizes human equality and unity before God, a procession around the Kaaba and the symbolic stoning of evil. Hide Caption 8 of 8 
 
 Known as the fifth pillar of Islam, the Hajj is an obligation upon every Muslim who has the financial means and the physical ability to perform it. For most, it is the spiritual climax of their lives, with many saving for decades to make the journey. ||||| Media playback is unsupported on your device Media caption This pilgrim in Mina says she is "very scared" after Thursday's crush 
 
 Saudi Arabia's King Salman has ordered a safety review for the Hajj pilgrimage after at least 717 people died in a stampede near the holy city of Mecca. 
 
 Another 863 people were injured in the incident at Mina, which occurred as two million pilgrims were taking part in the Hajj's last major rite. 
 
 It is the deadliest incident to occur during the pilgrimage in 25 years. 
 
 The king said there was a need "to improve the level of organisation and management of movement" of pilgrims. 
 
 In pictures: Aftermath of the stampede 
 
 Accounts from BBC staff in Mina 
 
 Hajj's safety concerns 
 
 'It is not God's will, it is man's incompetence' 
 
 The crush occurred after two massive lines of pilgrims converged on each other from different direction at an intersection close to the five-storey Jamarat Bridge in Mina. 
 
 Media playback is unsupported on your device Media caption BBC reporter Tchima Illa Issoufou: "I lost my aunt as a result of the stampede" 
 
 As part of the Hajj, pilgrims travel to Mina, a large valley about 5km (3 miles) from Mecca, to throw seven stones at pillars called Jamarat, which represent the devil. The pillars stand where Satan is believed to have tempted the Prophet Abraham. 
 
 The crush is the second disaster to strike in two weeks, after a crane collapsed at the Grand Mosque in Mecca, killing 109 people. 
 
 Why do millions gather in Mecca every year? 
 
 Iranian anger 
 
 Offering condolences to the relatives of the dead and injured, King Salman said: "We have instructed concerned authorities to review the operations plan and to raise the level of organisation and management to ensure that the guests of God perform their rituals in comfort and ease." 
 
 Saudi Arabia's Crown Prince Mohammed bin Nayyef, who chairs the Hajj committee, has begun an inquiry into the tragedy. 
 
 Deaths reported so far by nationality 
 
 Iran: 131 
 
 India: 14 
 
 Pakistan: 6 
 
 Turkey: 4 
 
 Indonesia: 3 
 
 Kenya: 3 
 
 Other nationalities (numbers not yet known): Niger: Chad 
 
 Interior ministry spokesman, Maj Gen Mansour al-Turki, said the reason for the unusual number of pilgrims at the site of the disaster was "not known yet". 
 
 Health Minister Khaled al-Falih promised a "fast" investigation and said the crush occurred "perhaps because some pilgrims moved without following instructions by the relevant authorities". 
 
 However, the head of Nigeria's Hajj delegation, the Emir of Kano Muhammadu Sanusi II, told the BBC the crush "happened on the designated ways for incoming and outgoing pilgrims to the site crossing each other - which shouldn't be. 
 
 "We are therefore urging the Saudi authorities not to apportion blame to the pilgrims for not obeying instructions." 
 
 Iran has fiercely criticised Saudi Arabia's handling of the pilgrimage. 
 
 Announcing three days of national mourning, Supreme Leader Ayatollah Ali Khamenei said: "The Saudi government should accept the responsibility of this sorrowful incident... Mismanagement and improper actions have caused this catastrophe." 
 
 'Tripping over' 
 
 The disaster began at 09:00 local time (06:00 GMT) on Thursday. 
 
 Gen Turki said: "The great heat and fatigue of the pilgrims contributed to the large number of victims." 
 
 The temperature in Mina was 46C on Thursday. 
 
 Image copyright AFP/Getty Images Image caption "Dead bodies stretch as far as my eyes can see," one BBC employee in Mina said 
 
 Image copyright SPA Image caption Hundreds of thousands of people continued to the Jamarat pillars despite the tragedy earlier 
 
 Image copyright AP Image caption Pilgrims converge on Mina to cast stones at three pillars representing the devil 
 
 "I saw someone trip over someone in a wheelchair and several people tripping over him," Abdullah Lotfy, from Egypt, told the Associated Press. "People were climbing over one another just to breathe." 
 
 Photographs showed the bodies of dozens of pilgrims on the ground, some piled high. They were all dressed in the simple white garments worn during the Hajj. 
 
 "Dead bodies stretch as far as my eyes can see," said Bashir Sa'ad Abdullahi, the BBC's Abuja editor, who is in Mina. 
 
 The UK Foreign Office said it was urgently seeking more information about whether British nationals were involved. 
 
 Pope Francis, who is visiting the US, expressed his "sentiments of closeness" with Muslims, during a prayer service at St Patrick's Cathedral in New York. 
 
 Hajj: Previous tragedies 
 
 2006: 364 pilgrims die in a crush at foot of Jamarat Bridge in Mina 
 
 1997: 340 pilgrims are killed when fire fuelled by high winds sweeps through Mina's tent city 
 
 1994: 270 pilgrims die in a stampede during the stoning ritual 
 
 1990: 1,426 pilgrims, mainly Asian, die in a stampede in an overcrowded tunnel leading to holy sites 
 
 1987: 402 people die when security forces break up an anti-US demonstration by Iranian pilgrims 
 
 Timeline: Deadliest stampedes 
 
 The Saudi authorities have spent billions of dollars on improving transport and other infrastructure to try to prevent such incidents. 
 
 The Hajj is the fifth and final pillar of Islam. It is the journey that every able-bodied adult Muslim must undertake at least once in their lives if they can afford it. 
 
 The number of people attending Hajj rose from 57,000 in 1921 to a high of 3.2m three years ago, according to the Saudi Central Department of Statistics and Information. 
 
 That figure dropped to just over two million last year. 
 
 Are you at Mecca, or do you know someone who is? If it is safe to do so, let us know about your experiences. Email haveyoursay@bbc.co.uk with your stories. 
 
 Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: ||||| Media playback is unsupported on your device Media caption BBC reporter Tchima Illa Issoufou: "I lost my aunt as a result of the stampede" 
 
 At least 717 people taking part in the Hajj pilgrimage have been killed in a stampede near the Islamic holy city of Mecca, officials in Saudi Arabia say. 
 
 Another 863 people were injured in the incident at Mina, which occurred as two million pilgrims were taking part in the Hajj's last major rite. 
 
 It is the deadliest incident to occur during the pilgrimage in 25 years. 
 
 King Salman has ordered an urgent review of this year's Hajj plans "to improve the level of organisation". 
 
 The supreme leader of Iran, which lost at least 95 of its citizens in the crush, has criticised Saudi Arabia's preparedness. 
 
 It is the second disaster to strike Mecca in two weeks, after a crane collapsed at the Grand Mosque, killing 109 people. 
 
 In pictures: Aftermath of the stampede 
 
 Accounts from BBC staff in Mina 
 
 During the Hajj, pilgrims travel to Mina, a large valley about 5km (3 miles) from Mecca, to throw seven stones at pillars called Jamarat, which represent the devil. 
 
 The pillars stand where Satan is believed to have tempted the Prophet Abraham. 
 
 At the scene - Tchima Illa Issoufou, BBC Hausa 
 
 Image copyright AFP/Getty Images 
 
 People were going towards the direction of throwing the stones while others were coming from the opposite direction. Then it became chaotic and suddenly people started going down. 
 
 There were people from Nigeria, Niger, Chad and Senegal among other nationalities. People were just climbing on top of others in order to move to a safer place and that's how some people died. 
 
 People were chanting Allah's name while others were crying, including children and infants. People fell on the ground seeking help but there was no-one to give them a helping hand. Everybody seemed to be on their own. 
 
 It affected some members of our group. I lost my aunt as a result of the stampede and at the moment, two women from our entourage - a mother and her daughter - are still missing. 
 
 The Saudi civil defence directorate said the stampede occurred at around 09:00 local time (06:00 GMT) as pilgrims were walking towards the five-storey structure which surrounds the pillars, known as the Jamarat Bridge. 
 
 Maj Gen Mansour al-Turki, a spokesman for the Saudi interior ministry, said the crush occurred when two large groups of pilgrims converged from different directions on to one street. 
 
 Photographs showed the bodies of dozens of pilgrims on the ground, some piled high. They were all dressed in the simple white garments worn during the Hajj. 
 
 Image copyright AFP/Getty Images Image caption "Dead bodies stretch as far as my eyes can see," one BBC employee in Mina said 
 
 Image copyright AP Image caption Dozens of Iranians were reportedly among the dead 
 
 Image copyright AFP Image caption The incident occurred as pilgrims walked towards the Jamarat Bridge, where the pillars are located 
 
 "I saw someone trip over someone in a wheelchair and several people tripping over him," Abdullah Lotfy, from Egypt, told Associated Press. "People were climbing over one another just to breathe." 
 
 "Dead bodies stretch as far as my eyes can see," said Bashir Sa'ad Abdullahi, the BBC's Abuja editor, who is in Mina. 
 
 The civil defence directorate said the victims were of "different nationalities", without providing details. 
 
 As well as victims from Niger witnessed by the BBC's correspondent at the scene, Iran's state news agency, Irna, said at least 95 Iranians were among the dead, with another 150 injured. 
 
 The BBC understands at least three Indonesians are among the dead. 
 
 Hajj: Previous tragedies 
 
 2006: 364 pilgrims die in a crush at foot of Jamarat Bridge in Mina 
 
 1997: 340 pilgrims are killed when fire fuelled by high winds sweeps through Mina's tent city 
 
 1994: 270 pilgrims die in a stampede during the stoning ritual 
 
 1990: 1,426 pilgrims, mainly Asian, die in a stampede in an overcrowded tunnel leading to holy sites 
 
 1987: 402 people die when security forces break up an anti-US demonstration by Iranian pilgrims 
 
 Timeline: Deadliest stampedes 
 
 Saudi Crown Prince Mohammed bin Nayef, who headed an emergency meeting after the stampede, has ordered an investigation. 
 
 The Saudi health minister, Khaled al-Falih, said the crush occurred because many pilgrims moved "without respecting the timetables" established by authorities. 
 
 Image copyright AP Image caption Pilgrims converge on Mina to cast stones at three pillars representing the devil 
 
 Saudi-owned al-Arabiya TV reported that the head of the central Hajj committee, Prince Khaled al-Faisal, blamed the stampede on "some pilgrims with African nationalities". 
 
 But Ayatollah Ali Khamenei, Iran's supreme leader, said the Saudi government "must accept the huge responsibility for this catastrophe", adding that "mismanagement and improper actions" were to blame. 
 
 The UK Foreign Office said it was urgently seeking more information about whether British nationals were involved. 
 
 What happens at the Hajj? 
 
 Image copyright AP 
 
 What rituals do pilgrims perform? The pilgrimage takes place in several stages over five days, including circling the Kaaba (a cube-like building in the centre of Mecca's Grand Mosque) en masse and throwing seven stones at pillars called Jamarat which represent the devil. 
 
 The pilgrimage takes place in several stages over five days, including circling the Kaaba (a cube-like building in the centre of Mecca's Grand Mosque) en masse and throwing seven stones at pillars called Jamarat which represent the devil. How many people go? Well over a million pilgrims from outside Saudi Arabia, and several hundred thousand from inside the kingdom, converge on the site each year. 
 
 Well over a million pilgrims from outside Saudi Arabia, and several hundred thousand from inside the kingdom, converge on the site each year. How do the authorities cope? Authorities deployed 100,000 security personnel and 25,000 extra health workers this year, as well as 100,000 air-conditioned tents for temporary accommodation. 
 
 Why do millions gather in Mecca every year? 
 
 The Saudi authorities have spent billions of dollars on improving transport and other infrastructure to try to prevent such incidents. 
 
 The Hajj is the fifth and final pillar of Islam. It is the journey that every able-bodied adult Muslim must undertake at least once in their lives if they can afford it. 
 
 The number of people attending Hajj rose from 57,000 in 1921 to a high of 3.2m three years ago, according to the Saudi Central Department of Statistics and Information. 
 
 That figure dropped to just over two million last year. ||||| CAIRO (AP) — Every year, millions of Muslims converge on the Saudi holy cities of Mecca and Medina for the annual hajj pilgrimage, with the massive ceremonies representing a major security and logistical challenge for the kingdom's authorities. 
 
 FILE - In this Jan. 5, 2006 file photo, rescuers attend to the scene of a building collapse in Mecca, Saudi Arabia. The day before the hajj began, an eight-story building being used as a hostel near the... (Associated Press) 
 
 FILE - In this April 9, 1998 file photo, pilgrims walk past a broken umbrella left on the ground at the site of a stampede that left more than 150 people dead at Mina, Saudi Arabia. The stampede broke... (Associated Press) 
 
 In this image posted on the official Twitter account of the directorate of the Saudi Civil Defense agency, rescuers respond to a stampede that killed and injured pilgrims in the holy city of Mina during... (Associated Press) 
 
 On occasion, the hajj and events surrounding it have been marred by accidents and tragedies, such as Thursday's stampede near Mecca that killed at least 310 people. 
 
 Here's a look at some deadly hajj-related incidents: 
 
 ___ 
 
 2015: At least 310 people are killed and hundreds injured in a stampede in Mina, on the outskirts of the holy city of Mecca. In the lead-up to hajj, 111 people are killed and nearly 400 injured when a crane collapses in bad weather, crashing onto the Grand Mosque in Mecca, Islam's holiest site. 
 
 ___ 
 
 2006: More than 360 pilgrims are killed in a stampede at the desert plain of Mina, near Mecca, where pilgrims carry out a symbolic stoning of the devil by throwing pebbles against three stone walls. The day before the hajj began, an eight-story building being used as a hostel near the Grand Mosque in Mecca collapsed, killing at least 73 people. 
 
 ___ 
 
 2004: A crush of pilgrims at Mina kills 244 pilgrims and injures hundreds on the final day of the hajj ceremonies. 
 
 ___ 
 
 2001: A stampede at Mina during the final day of the pilgrimage ceremonies kills 35 hajj pilgrims. 
 
 ___ 
 
 1998: About 180 pilgrims are trampled to death in panic after several of them fell off an overpass during the final stoning ritual at Mina. 
 
 ___ 
 
 1997: At least 340 pilgrims are killed in a fire at the tent city of Mina as the blaze was aided by high winds. More than 1,500 were injured. 
 
 ___ 
 
 1994: Some 270 pilgrims are killed in a stampede during the stoning ritual at Mina. 
 
 ___ 
 
 1990: The worst hajj-related tragedy claims the lives of 1,426 pilgrims in a stampede in an overcrowded pedestrian tunnel leading to holy sites in Mecca.
– A stampede at the "stoning the devil" ritual has become the hajj pilgrimage's worst disaster in a decade. Saudi authorities say at least 717 people were killed; previous reports said at least another 450 were injured in the crush of people at Mina, near the holy city of Mecca, the AP reports. The BBC explains that the hajj is "the fifth and final pillar of Islam"; every adult Muslim who can physically and financially manage to do so is expected to make the pilgrimage once in their lives. Two million people are this year participating in the annual five-day pilgrimage, which started Tuesday. One of the final major rites of the hajj involves casting stones at pillars that signify the devil. After numerous similar disasters at the ritual, including a 2006 stampede that killed 345 people, the Saudis spent $1.2 billion on measures to reduce congestion at the site where pilgrims hurl stones at three pillars, CNN reports. Read about other hajj-related tragedies, the worst of which happened in 1990, here.