The Syrian Observatory for Human Rights says civilians were killed in Douma, the largest town of the opposition-held enclave near Damascus.
At least 20 civilians have died in a resumption of bombing on Douma, the largest town in shrinking rebel-controlled pockets of Syria's eastern Ghouta, a monitor said on Monday.
Thirteen were killed late on Sunday in air strikes and artillery fire on the battered town, and another seven were killed early on Monday, according to the Syrian Observatory for Human Rights.
The fresh bloodshed came after a week-long lull in the bombardment of Douma after negotiations between rebels and regime-backer Russia allowed medical evacuations from the town.
After the bombing an AFP correspondent saw two rescue workers scouring mountains of rubble in the dark with tiny flashlights, searching for survivors.
A third, lifted a wounded person, who was screaming in pain, onto his back.
As morning broke, the sounds of shelling could still be heard across the city.
Entire buildings had been gutted by strikes, with a drying rack, glass, and plastic piping spilling out into the streets.
Several members of a single family remained stuck underneath a collapsed building all night, including a handicapped mother, her son and her grandson, AFP 's correspondent said.
Syria's regime has pressed a ferocious month-long air and ground assault on eastern Ghouta in a bid to clear the last opposition bastion on the capital's outskirts.
More than 1,400 civilians including 281 children have been killed, according to the Britain-based Observatory.
Regime troops have captured more than 80 percent of the enclave and split the remaining opoosition territory into three pockets, with Douma in the northernmost zone.
Talks between regime ally Russia and Jaish al Islam, the opposition group that holds Douma, had resulted in a brief respite for the town.
During the lull, hundreds of people including the sick and wounded were evacuated from Douma to regime-controlled territory.
Syrian regime troops and allied militia are also unleashing aggression on other parts of the enclave, with regime air strikes killing five civilians in another zone on Monday, according to the Observatory.
The deadly raids came as more than 4,000 civilians fled a southern pocket of eastern Ghouta.
The Observatory says nearly 70,000 people have escaped eastern Ghouta in recent days to regime-held zones, using "corridors" opened by the regime army.