Media playback is unsupported on your device Media caption On Sunday, Turkish troops fired tear gas at the Syrian border, as Mark Lowen reports 
 
 Turkey has begun to close some of its border crossings with Syria after about 130,000 Kurdish refugees entered the country over the weekend. 
 
 On Sunday Turkish security forces clashed with Kurds protesting in solidarity with the refugees. Some protesters were reportedly trying to go to Syria to fight Islamic State (IS). 
 
 Most refugees are from Kobane, a town threatened by the advancing militants. 
 
 IS has taken over large swathes of Iraq and Syria in recent months. 
 
 Before the latest influx, there were already more than one million Syrian refugees in Turkey. They have fled since the start of the uprising against Syrian President Bashar al-Assad three years ago. 
 
 Some of the new arrivals are being sheltered in overcrowded schools, as Turkey struggles to cope with the influx. 
 
 'Huge numbers' 
 
 On Friday Turkey opened a 30km (19-mile) section of the border to Syrians fleeing the town of Kobane, also known as Ayn al-Arab. 
 
 But on Monday only two out of nine border posts in the area remained open, the UN refugee agency UNHCR said. 
 
 Mark Lowen, BBC News, southern Turkey 
 
 Turkey was already struggling to cope with over a million Syrian refugees - that was before 130,000 more fled across the border at the weekend. The sheer numbers would overwhelm any country. 
 
 Added to that, they are Kurds from Syria, many of them deeply hostile to Turkey. For 30 years, Turkish forces fought Kurdish rebels in a civil war that killed 40,000 people. 
 
 The fact that Turkey is accepting tens of thousands of Kurds is a sign of how allegiances are being forced to change with the onslaught of Islamic State. But deep-seated tension between Kurds and Turks have again come to the surface, leading to Sunday's border clashes. 
 
 Turkey fears that Kurds will cross into Syria to join the Kurdish militia. The worry is that, renewed by fresh recruits, it could ally with the outlawed PKK and launch attacks on Turkish soil. All the regional complexities, added to the refugee influx, make for a precarious situation here. 
 
 Clashes broke out on Sunday after a demonstration by Kurds on the Turkish side of the border. 
 
 Some protesters threw stones at security forces, who responded with tear gas and water cannon. There were no reports of serious injuries. 
 
 Turkish security forces were trying to stop Kurdish fighters from entering Syria to take part in the defence of Kobane, says the BBC's Mark Lowen at the scene. 
 
 The Kurdistan Workers' Party (PKK), a banned militant group that fought a civil war for autonomy within Turkey for decades, has called on Kurds to join the fight against IS. 
 
 Our correspondent says the Syrian conflict has reawakened old hostilities and shaken a fragile peace between Kurds and Turkish authorities. 
 
 PKK-affiliated forces have been battling IS in northern Iraq for months. 
 
 Image copyright Reuters Image caption Most of the border crossings from Syria to Turkey are now closed 
 
 Image copyright AP Image caption Turkish security forces clashed with demonstrators at one crossing on Sunday 
 
 Carol Batchelor, a spokeswoman for the UN refugee agency in Turkey, told the BBC that the country needed urgent aid to cope with the influx - including food, blankets and winter clothing, "particularly for children". 
 
 In other developments: 
 
 A Syrian government air strike killed at least 42 civilians in rebel-held territory in the north-western province of Idlib on Sunday, activists said 
 
 IS issued a statement urging its supporters to kill "disbelievers" from the countries that have joined a international coalition against the group 
 
 US Secretary of State John Kerry and Iranian counterpart Mohammad Javad Zarif discussed the threat posed by IS during a meeting in New York on Sunday, a US official said 
 
 IS is closing in on the predominantly Kurdish town of Kobane, having seized dozens of villages in the area in recent days. 
 
 It began the assault on Tuesday, and by Sunday militants were about 10km (six miles) away, said the Syrian Observatory for Human Rights. 
 
 Reports suggest that IS has used heavy weaponry, including tanks, in the attack. 
 
 The US has said it will attack the group in Syria as part of a strategy to destroy it, though so far it has carried out air strikes against IS only in Iraq. 
 
 Media playback is unsupported on your device Media caption Tony Blair: " Isis... kill without mercy and they're prepared to die without regret" 
 
 Attacking IS in Syria is considered more complicated, partly because of the strength of the country's air defence system and because foreign strikes do not have the approval of President Assad. 
 
 Former UK Prime Minister Tony Blair told the BBC on Sunday that air strikes alone might not be enough to contain the group. 
 
 "If necessary, we shouldn't rule out the use of, particularly, special force capabilities," he said. 
 
 President Obama has previously ruled out the involvement of US ground troops, and has instead promised to provide arms and training to local forces fighting against IS. 
 
 Who are Islamic State (IS)? ||||| A UN official warned of pending "humanitarian tragedies" and pleaded desperately with the world to intervene on behalf of Kurds trapped in a Syrian city near the Turkish border, as Islamic State fighters stood on the brink of taking it. 
 
 Kurds from villages throughout northern Syria have fled to Kobani for a final stand as the terrorist group has marauded across huge swaths of land, leaving a trail of death and destruction. With the city under siege for three weeks, the black-clad fighters have begun to raise their flag over neighborhoods and UN Special Envoy for Syria Staffan de Mistura said the city was about to fall. 
 
 "The world has seen with its own eyes the images of what happens when a city in Syria or in Iraq is overtaken by the terrorist group called ISIS or Da'esh: massacres, humanitarian tragedies, rapes, horrific violence," De Mistura said. "The international community cannot sustain another city falling under ISIS. 
 
 "The world, all of us, will regret deeply if ISIS is able to take over a city which has defended itself with courage but is close to not being able to do so," De Mistura added. "We need to act now." 
 
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 With the fighting taking place with view of the Turkish border, international pressure increased on Ankara to get involved militarily. Turkey has already taken in an estimated 200,000 refugees, and Turkish President Recep Tayyip Erdogan said the coalition air campaign launched last month would not be enough to halt the Islamic State group's advance. 
 
 "Kobani is about to fall," Erdogan told Syrian refugees in the Turkish border town of Gaziantep, according to The Associated Press. 
 
 Erdogan, whose troops are massed near the border but have so far not taken an offensive posture, called for greater cooperation with the Syrian opposition, which is fighting both the extremists and forces loyal to Syrian President Bashar Assad. 
 
 "We asked for three things: one, for a no-fly zone to be created; two, for a secure zone parallel to the region to be declared; and for the moderate opposition in Syria and Iraq to be trained and equipped." 
 
 The capture of Kobani would give ISIS control of a large swath of land bordering Turkey and eliminate a vital pocket of Kurdish resistance. It would also provide a link between the group's territory near the ancient Syrian city of Aleppo and its largest operations base at Raqqa in northeastern Syria. 
 
 The Associated Press reported that warplanes believed to be part of the U.S.-led coalition against Islamic State, struck militant positions Tuesday. Journalists on the Turkish side of the border heard the sound of warplanes before two large plumes of smoke billowed just west of Kobani. A Fox News crew on the Turkish side of the border reported only one U.S. airstrike in the previous five days. 
 
 Fighting continued into Tuesday morning on the outskirts of the town. One coordinator with the Kurdish defenders told The New York Times that their defenses benefited from the new round of airstrikes, but they were still outmatched by the more heavily armed militants. 
 
 On Tuesday morning, the AP reported that occasional gunfire could be heard in Kobani, also known by the Arabic name Ayn Arab. A flag of the Kurdish force known as the People's Protection Units, or YPG, was seen flying over a hill in the center of town. 
 
 The Wall Street Journal reported that ISIS fighters had entered the eastern outskirts of the city on Monday after capturing more than 300 surrounding Syrian Kurdish villages in the previous three weeks. The paper also reported that the militants raised their black flag in two separate places, one on top of a civilian apartment building and another on a hilltop near a checkpoint at the city’s eastern entrance. The flag at the checkpoint could be seen by reporters watching from across the border in Turkey. 
 
 The Britain-based Syrian Observatory for Human Rights said the Kurds forced the jihadists to withdraw from the eastern part of the town in heavy clashes after midnight Tuesday, adding that five loud explosions were heard in the town as warplanes soared overhead. However, a local Kurdish militia commander estimated to The Journal that ISIS fighters were still a mile from the city center. On Tuesday, Reuters reported that ISIS had also taken over several buildings in the southwest of the city. 
 
 Before the recent fighting began, the city had been a focal point for refugees fleeing Syria's three-years-long civil war. Between 160,000 and 180,000 people are believed to have fled into Turkey since the ISIS advance began. A Kurdish politician told Reuters that more than 2,000 Syrian Kurds, including women and children had been evacuated from the town in the midst of the fighting Monday. 
 
 State Department officials told the Journal that U.S. officials will travel to Turkey later this week to discuss the status of the international coalition. Retired Marine Gen. John Allen, the White House's special envoy in the fight against ISIS, is among those traveling to Turkey. 
 
 Despite U.S. pressure to become a full-fledged member of the coalition, and despite the Turkish parliament passing a law giving the government authority to conduct operations against ISIS in Syria or Iraq, Ankara has largely stayed on the sidelines. Fox News reported Monday that twenty Turkish tanks have been stationed on a hillside overlooking Kobani, ready to strike the city on short notice. However, Turkish authorities have mostly been preoccupied with attempting to control the flow of Kurdish refugees across the border and deal with their protests at the government's inaction. 
 
 Also Tuesday, Turkish media reported that police in Istanbul and at least six other Turkish cities clashed with hundreds of demonstrators. The private Dogan news agency clashes broke out in several Istanbul neighborhoods overnight, as protesters set up barricades, hurled stones, fireworks and firebombs at police and set a bus on fire. One police officer was injured. 
 
 Police used tear gas and water cannon to disperse similar protests in the mostly Kurdish-populated cities of Diyarbakir, Batman, Van, Sirnak, Sanliurfa and Hakkari. 
 
 Fox News' Greg Palkot and The Associated Press contributed to this report.
– Up until the middle of last week, Turkey put the number of Syrian refugees within its borders at more than a million. That number has swelled by 130,000 in the last four days, according to Turkey's deputy PM. It's not just the sheer number of refugees that's noteworthy: They're mainly Kurds, and as the BBC reports, many are "deeply hostile" to Turkey. And since Thursday, they've been pouring into the country in an attempt to flee ISIS militants who have, as the AP puts it, "pushed the conflict nearly within eyeshot of the Turkish border." The current conflict centers around the town of Kobani, which ISIS attempted to take in July, reports Reuters. That attempt was quelled with the help of Kurds who entered Syria from Turkey, and there were skirmishes on the Turkish side of the border yesterday as the country tried to prevent Kurds from flowing into Syria once again. As BBC correspondent Mark Lowen explains, there's a fear that the Kurdistan Workers' Party, which for decades battled for autonomy for Kurds inside Turkey, could team up with a beefed-up Syrian Kurdish militia and once again take up arms within Turkey. For now, the Syrian Kurdish fighters are said to have halted an ISIS advance on Kobani from the east, according to a rep for the Kurdish group, who says that's the front where the most intense fighting is taking place.