Syrian troops loyal to President Bashar al-Assad stormed a flashpoint town near Damascus on Sunday, said a watchdog, adding that scores of bodies, some of them disfigured, were found there.
The Syrian Observatory for Human Rights said at least 80 people have been killed in the town of Jdaidet al-Fadl in five days of violence.
"Regime troops have taken complete control over the town of Jdaidet al-Fadl and its surroundings," said the Observatory which has documented the names of at least 80 people killed.
Observatory director Rami Abdel Rahman told AFP he feared the death toll could be much higher, but that it was difficult to document the killings after the entry of regime forces into the town.
"We call on the International Committee of the Red Cross to send a delegation to Jdaidet al-Fadl in order to investigate," Abdel Rahman said.
Three of those killed were children and six others were women, the Observatory said, adding that the rest were men, among them many rebel fighters.
The Observatory distributed amateur video filmed in the town showing the bodies of men laid out on the ground, some with mutilated faces and covered in blood.
Jdaidet al-Fadl is located in an area southwest of Damascus that is home to several communities, including Sunnis, Druze and Christians.
After the outbreak of Syria's conflict, which the United Nations says has forced millions to flee their homes, many of those living in the area were internally displaced people.
An activist in Damascus said via the Internet that all communication, water and electricity have been cut off in Jdaidet al-Fadl, adding that some of the corpses were "found in one of the hospitals" in the town.
Identifying herself as Enas, the activist said some of the bodies "were severely beaten on the head, causing it to crack open. Others had bruises all over their face". She added that other bodies had been "burnt".
Since last year, the army has tried to root out rebels positioned southwest and east of Damascus, in a bid to secure the capital.
Sunday's violence comes a day after at least 121 people were killed across Syria, the Observatory reported.