Russian and Syrian warplanes killed at least 17 civilians in eastern Ghouta on Saturday, the Syrian Observatory for Human Rights.
Syrian government forces have been trying to reach a force trapped inside the suburb of Harasta for a week without success.
The Syrian state media reported an operation in Eastern Ghouta was ongoing meant to break the siege of a military site nearby, though it's not clear if the airstrikes were related.
Eight people died in the village of Hammuriye, while two others were killed in Medyere, and three civilians were also killed in Arbin.
Seven children and 11 women were among those killed, said the head of the war monitor, which relies on a network of sources inside Syria.
Residents including members of the White Helmets rescue group rushed to rescue the wounded.
More than 340,000 people have been killed in Syria and millions displaced since the conflict began in March 2011 with anti-government protests.
Despite eastern Ghouta being designated as a de-escalation zone, it has witnessed severe shelling and confrontations between regime forces and armed Syrian factions.
The latest fighting there comes as regime troops backed up by Russian airpower battle rebels and jihadis on the edge of northwestern Idlib province, the only one still fully beyond government control.
But hostilities intensified in mid-November, when the Syrian military stepped up air and artillery attacks on the enclave in response to a rebel offensive.
A government forces base, the only one in the region, was surrounded by rebels earlier this week.
Four bombs hit two buildings in a residential area, and the civilians were within.
In a separate development, the Russian defence ministry denied a report in the newspaper Kommersant that seven Russian military planes had been destroyed by rebel shelling on Sunday of the Hmeimim airbase, near Latakia.
The Tass news agency cited the defence ministry as saying that two personnel were killed by mortar fire but that no military hardware was lost.