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, and many people are too scared to leave.
Although the International Committee of the Red Cross (ICRC) and its Syrian Red Crescent partners have since been told they cannot enter the district because of the risk of booby-traps and mines, state television has reported that it has 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 could not 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.
Several men who said they had defected from an elite army unit last week told our correspondent that civilians were being targeted deliberately by security forces and their 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.’
“An old man was arrested. An officer put a gun to his head. He said: ‘This is for freedom,’ and he shot him.”