Media caption Victoria Nuland, US state department, says they fear the regime may be planning an attack on al-Haffa 
 
 The US has expressed fears that the Syrian government "may be organising another massacre" in the town of Haffa in Latakia province, where UN military observers have been denied access. 
 
 UN Secretary General Ban Ki-moon said it was vital that the observers should be allowed into the town. 
 
 He and peace envoy Kofi Annan expressed grave concern about violence there. 
 
 A BBC correspondent travelling with UN observers witnessed sustained and heavy shelling in the old city of Homs. 
 
 The UN observers are in Syria to monitor the implementation of a putative ceasefire, brokered by Mr Annan, which has all but collapsed. 
 
 'Civilians trapped' 
 
 Annan's six-point plan 1. Syrian-led political process to address the aspirations and concerns of the Syrian people 2. End to violence by all sides; army troops to stop using heavy weapons and withdraw to barracks 3. Parties to allow humanitarian aid 4. Authorities to free political detainees 5. Authorities to ensure freedom of movement for journalists 6. Authorities to allow peaceful demonstrations 
 
 "The UN made a particular appeal to be able to get into Haffa because it had heard concerning reports about regime plans," US state department spokeswoman Victoria Nuland told a news briefing in Washington. 
 
 "At the same time, if you follow Syrian media, they were claiming that the opposition would cause a massacre." 
 
 Earlier this month, activists said Syrian government forces killed 108 people in the region of Houla, in Homs province, and 78 people in the village of Qubair, in Hama province. 
 
 But Ms Nuland downplayed the idea of foreign intervention in Syria, saying the emphasis would remain on using political and economic measures to pressure the government of President Bashar al-Assad. 
 
 The Syrian government blames the violence on foreign-backed armed terrorist gangs. 
 
 In a statement, Mr Annan, joint envoy for the UN and Arab League, said there were "indications that a large number of civilians are trapped" in Homs and Haffa. 
 
 Later Mr Ban spoke of the "great danger" to civilians under fire. 
 
 "The secretary general underlines the importance of unimpeded access by UNSMIS to Haffa, amid reports of a build-up of government forces around the town," he said in a statement. 
 
 Syrian government helicopter gunships strafed rebel positions in Haffa, UN observers and human rights activists said. 
 
 Media caption The BBC's Paul Danahar: "I was watching a mortar land pretty much every minute" 
 
 Heavy fighting was also reported in Rastan and Talbisa, north of Homs city, "with artillery and mortar shelling, as well as firing from helicopters, machine guns and smaller arms", said a statement from the UN Supervision Mission in Syria (Unsmis). 
 
 A car bomb exploded in the city of Deir al-Zour, killing 10 people, the London-based Syrian Observatory for Human Rights said, quoting sources on the ground. 
 
 Fighting was also reported in Idlib province. 
 
 It is impossible to confirm these reports independently, as Syria heavily restricts journalists' freedom of movement. 
 
 Russia, which supports the Syrian government, called for Iran to take a role in ending the 15-month old conflict. 
 
 Last week, US Secretary of State Hillary Clinton rejected the idea of involving Iran, which she accused of "helping to stage-manage the repression" in Syria. 
 
 Russian Foreign Minister Sergei Lavrov will travel to Tehran on Wednesday to discuss the initiative, the foreign ministry said. ||||| Media playback is unsupported on your device Media caption One woman told Paul Wood how two of her brothers were detained, and one was killed 
 
 People fleeing the central Syrian city of Homs have told the BBC that security forces are committing atrocities there. 
 
 One woman told the BBC's Paul Wood on the outskirts of Homs that soldiers had slit the throat of her 12-year-old son on Friday - a day after rebel fighters withdrew from the Baba Amr district. 
 
 She said 35 other men and boys from her area had also been detained and killed. 
 
 The government has denied the Red Cross access to Baba Amr for four consecutive days, citing security concerns. 
 
 Activists have warned of a humanitarian catastrophe. 
 
 Electricity, water and communications have been cut off, and in recent days temperatures have plummeted and snow has fallen. Food supplies are said to be dangerously low. 
 
 'Screams' 
 
 On Thursday, government troops backed by tanks entered Baba Amr after the rebel Free Syrian Army (FSA) announced a "tactical withdrawal". 
 
 Eyewitness A terrible fear has seized people here about what the government forces are doing now that they are back in control. In a house we sat with six women and their 17 children. They had arrived that day. There were no men. "We were walking out altogether until we reached the checkpoint," said one of the women, Um Abdo. "Then they separated us from the men. They put hoods on their heads and took them away." Where do you think they are now, I asked? The women replied all at once: "They will be slaughtered." Read Paul's report in full 
 
 The International Committee of the Red Cross (ICRC) and its Syrian Red Crescent partners have been told they cannot enter the district because of the risk of booby-traps and mines, although state TV reported it had been "sanitised" of "armed terrorist groups". 
 
 Opposition and human rights activists have said security forces and pro-government militia have been rounding up men and boys over the age of 14 who are still in Baba Amr, and then torturing and killing them. 
 
 The claims cannot be substantiated, but people fleeing Homs also told our correspondent that security forces had been committing atrocities, including summary executions and cutting the throats of prisoners. 
 
 One woman, who had walked for three days to escape, said that on Friday troops had taken 36 men and boys from one area and killed them. 
 
 "My son's throat was cut," she said. "He was 12." 
 
 Her husband said he was hiding about 50m away and saw one soldier hold down their son's head with his boot while another killed him. 
 
 "I could hear their screams," he added. 
 
 Another woman said: "They took our husbands. They took them at a checkpoint. They will slaughter them like sheep." 
 
 We were told in this operation: 'You shoot anything that moves. Civilian or military - you shoot at it Syrian army defector Why BBC journalists risked visiting Homs 
 
 Several men who said they had defected from an elite army unit last week told our correspondent that civilians were being targeted by security forces and prisoners were being killed. 
 
 "A lieutenant gave us the order," he said. "We were told in this operation: 'You shoot anything that moves. Civilian or military - you shoot at it.'" 
 
 Our correspondent says the people of Baba Amr defied the government and now they are scattered, their uprising crushed. 
 
 The UK's Channel 4 News broadcast secretly shot footage on Monday that it said shows hospital patients in Homs being tortured by medical staff. 
 
 Pictures showed wards full of wounded men, shackled to their beds and blindfolded and some showing the marks of severe beatings. 
 
 The authorities have not commented and the video cannot be independently verified. 
 
 The UN's High Commissioner for Human Rights Navi Pillay told Channel 4 that the images accorded with other evidence gathered by a UN-backed commission of inquiry of torture in Syrian hospitals, particularly military hospitals. 
 
 She said there was evidence of similar occurrences in hospitals in Hama and Deraa. 
 
 The independent commission of inquiry said in February that Syrian security forces had "committed widespread, systematic and gross human rights violations, amounting to crimes against humanity, with the apparent knowledge and consent of the highest levels of the state". 
 
 Image caption Russia is a key Syrian ally and the largest supplier of arms to the Assad regime 
 
 The EU has said it will document alleged war crimes to set the stage for a "day of reckoning" for Syria's leaders. But Russia and China have vetoed two UN Security Council resolutions critical of the government. 
 
 UN humanitarian chief Valerie Amos said on Monday that Syria had approved her visit to the country and she planned to travel there on Wednesday "to urge all parties to allow unhindered access for humanitarian aid". 
 
 Former UN Secretary-General Kofi Annan is also due to visit Syria at the weekend as joint special envoy for the UN and the Arab League. 
 
 'Extremely difficult' 
 
 The director-general of the ICRC, Yves Daccord, earlier spoke of his concern about the humanitarian situation in Baba Amr and the need to gain access. 
 
 "We have to be firm and not give up," he said. "Negotiations are being led on site in Homs with military commanders and also in Damascus." 
 
 Media playback is unsupported on your device Media caption Carla Haddad Mardini, ICRC: "Today, four trucks of medical aid, food, blankets, have arrived in Homs" 
 
 A second ICRC convoy carrying food and other relief supplies arrived in Homs from Damascus on Monday. 
 
 Diplomatic pressure appeared to be growing on Russia on Monday to drop its support for President Bashar al-Assad. 
 
 German Foreign Minister Guido Westerwelle said he hoped Moscow would "see that it stands on the wrong side of history" in regard to Syria. 
 
 Meanwhile, the body of US journalist Marie Colvin is due to be flown back to New York on Tuesday morning. 
 
 Colvin, who worked for the Sunday Times, died in a rocket attack in Baba Amr on 22 February with French photographer Remi Ochlik.
– The US and UN are warning that the Syrian government may be organizing another massacre, this time in the town of Haffa, where UN monitors have been impeded, reports the BBC. International envoy Kofi Annan says he is "gravely concerned" about the escalation of violence, and reports of mortar, tank, and helicopter attacks on Haffa and its surrounding villages. "A large number of civilians are trapped in these towns," he warned. The Syrian regime is using "new horrific tactics" on rebellious areas, a State Department spokeswoman says, warning: "People will be held accountable." A new UN reports has, for the first time, placed the Syrian government and its allied militias on a list of groups that kill and sexually attack children, AP reports. The report found that in Syria, children as young as nine have been the victims of killing, torture, sexual violence, and arbitrary detention, and have been used as human shields.