BBC
A resident of the opposition stronghold of Homs told the BBC that soldiers had slit the throat of her 12-year-old son.
Meanwhile, the UN humanitarian chief said she had now been allowed to travel to Syria and would call for "unhindered access for humanitarian aid".
Valerie Amos said she planned to go to Syria on Wednesday.
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. On Wednesday, he will hold talks with league officials in Cairo.
'Screams' The UN refugee agency (UNHCR) said on Monday that many Syrian refugees - including women and children - had only a few belongings as they arrived into Lebanon.
"Those who can flee, do. Those who can't will die sitting down," she added.
People fleeing the city of Homs, 15km (nine miles) from al-Qusair, 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 killed her son on Friday - a day after rebel fighters withdrew from the Baba Amr district.
"My son's throat was cut," she said. "He was 12."
Her husband said he was hiding about 50m (160ft) 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.
The woman said 35 other men and boys from her area had also been detained and killed.
The Syrian government has denied the Red Cross access to Baba Amr district for four consecutive days, citing security concerns.
Activists have warned of a humanitarian catastrophe.
'Hospital torture' 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.
An independent commission of inquiry set up by the UN 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".
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.
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.
One woman told Paul Wood how two of her brothers were detained, and one was killed
Thousands of Syrians have recently crossed into Lebanon, the UN says, amid reports that security forces are committing atrocities in Syria.
The UN refugee agency said as many as 2,000 people fled in the past two days.A resident of the opposition stronghold of Homs told the BBC that soldiers had slit the throat of her 12-year-old son.
Meanwhile, the UN humanitarian chief said she had now been allowed to travel to Syria and would call for "unhindered access for humanitarian aid".
Valerie Amos said she planned to go to Syria on Wednesday.
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. On Wednesday, he will hold talks with league officials in Cairo.
'Screams' The UN refugee agency (UNHCR) said on Monday that many Syrian refugees - including women and children - had only a few belongings as they arrived into Lebanon.
Continue reading the main story 
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."
Eyewitness
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."
Residents of the northern Lebanese town of Arsal said that up 150 Syrian families arrived there on Sunday alone.
"What are we supposed to do? People are sitting in their homes and they are hitting us with tanks," Hassana Abu Firas, from Syria's border town of al-Qusair, told the Associated Press."Those who can flee, do. Those who can't will die sitting down," she added.
People fleeing the city of Homs, 15km (nine miles) from al-Qusair, 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 killed her son on Friday - a day after rebel fighters withdrew from the Baba Amr district.
"My son's throat was cut," she said. "He was 12."
Her husband said he was hiding about 50m (160ft) 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.
The woman said 35 other men and boys from her area had also been detained and killed.
Continue reading the main story 
“Start Quote
Syrian army defectorWe were told in this operation: 'You shoot anything that moves. Civilian or military - you shoot at it”
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 independently verified. The Syrian government has denied the Red Cross access to Baba Amr district for four consecutive days, citing security concerns.
Activists have warned of a humanitarian catastrophe.
'Hospital torture' 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.
An independent commission of inquiry set up by the UN 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".
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.
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.
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