People fleeing the central Syrian city of Homs have accused security forces of committing atrocities, including summary executions and cutting the throats of men and boys.
Troops and paramilitaries took full control of the Bab Amr quarter of Homs over the weekend after a month of bombardment.
The Syrian government has denied the Red Cross access to Baba Amr for four consecutive days, citing security concerns.
However activists said access was denied because troops were committing atrocities against the people left behind.
One woman told a BBC correspondent 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.
Activists have warned of a humanitarian catastrophe, as 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, and many people are too scared to leave.
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 could not be substantiated, but people fleeing Homs also told the BBC's correspondent that security forces had been committing atrocities.