More than 40 people, including six people reportedly summarily executed, were said to have been killed Thursday in a Damascus suburb, The Guardian newspaper reports.
The White House announced that Obama spoke by telephone with British Prime Minister David Cameron about the Syrian crisis.
"The two leaders exchanged views on ways the international community can assist those displaced by the conflict, apply pressure on the Assad regime and support the opposition so that the legitimate aspirations of the Syrian people can be realized," a White House statement read.
London this month announced it was supporting the Syrian opposition with nearly $8 million of non-lethal assistance.
The conflict in Syria has centered on the city of Aleppo, where rebel and government forces have fought for control, and in parts of Damascus.
A report this week by Amnesty International said civilians faced the worst of the violence during the fight for control over Aleppo. During a 10-day visit to the city in early August, the organization said more than 80 civilians who played no direct role in the conflict were killed during an estimated 30 attacks.
"In the overwhelming majority of cases the victims were killed or injured in attacks by government forces that violated international humanitarian law," Amnesty International said in a statement. "Some attacks were indiscriminate."