A record number of bodies was discovered Sunday in the Damascus suburbs of Jdaidet Artouz and Jdaidet al-Fadel, opposition activists said on Monday.
The Local Coordination Committees said the death toll was 483 and most of the people were killed in Jdaidet Artouz.
The result of a pitched battle between Free Syrian Army forces and loyalists of President Bashar Assad, the fighting in Damascus claimed the lives of some 300 civilians, Local Coordination Committees in Syria activists told CNN.
Sunday’s body count was the largest for a single day since the Syrian civil war broke out over two years ago, the activists said.
The UN in February estimated that the war has cost some 70,000 lives.
State-run news agency SANA said Syrian troops “inflicted heavy losses” on the rebels in the suburbs.
The US has long called for Assad to leave power, but for months did not play an active role in backing the rebellion.
Recently, however, Washington has grown more assertive, helping cobble together the Syrian National Coalition late last year in the hopes that it could unite the deeply divided opposition and provide a conduit for aid to rebels on the ground.
At a conference on Saturday in Istanbul that brought together the opposition leadership and its chief international supporters, US Secretary of State John Kerry announced that the Obama administration would double its non-lethal assistance to the Syrian opposition with an additional $123 million in supplies.
That could include for the first time armored vehicles, body armor, night vision goggles and other defensive military supplies, officials said.
The Associated Press contributed to this report.