Syrian opposition activists said on Sunday that government troops seized a town near Damascus, killing at least 80 people, including women and children.
The army stormed the town of Jdaidet al-Fadl after five days of heavy fighting, the activists said, according to the BBC.
Syria's SANA state news agency said government forces "inflicted heavy losses upon terrorists" in the town.
The Syrian Observatory for Human Rights, a UK-based activist group, said it was able to identify at least 80 people killed in Jdaidet al-Fadl, to the south-west of the capital.
The organization said some of the victims were summarily killed. There were reports of as many as 250 deaths, it told the BBC.
The Local Coordination Committees (LCC), another Syrian organization, said that Syrian forces committed a massacre after retaking the town.
Videos posted online showed rows of dead bodies, swaddled in blood-stained blankets. The LCC said hundreds were killed but there was no independent verification of that number.
On Saturday, the United States said it would double its aid to Syria's opposition, including with new non-lethal military equipment, but paid no heed to calls for arms supplies or a direct intervention.
In a statement after talks among the pro-opposition "Friends of Syria" group in Istanbul, Secretary of State John Kerry said U.S. assistance to the opposition would double to $250 million.
In response to the announcement, a Syrian government daily accused the United States on Sunday of having "thrown oil on the fire" of the country's conflict by doubling its aid to the opposition.
"The meeting of the so-called 'Friends of Syria' held in Istanbul yesterday has thrown oil on the fire of the Syrian conflict," said the Al-Watan daily.
Many in the West have raised concerns about arming the rebels, fearing weapons could end up in the hands of radical Islamist groups such as the Al-Nusra Front, which this month pledged allegiance to Al-Qaeda.
Western diplomats have said increased military support will hinge on the opposition showing it can be more inclusive and that it could ensure weapons would be secure. They would also have to reject the use of chemical weapons and guarantee respect for human rights.