A separate rocket attack on Sunday in a regime-held neighbourhood northeast of Damascus killed 11 people, state television said.
"There are now 34 civilians that were killed in Saturday's attacks on Douma, among them 12 children and eight women," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
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Forces loyal to President Bashar al Assad targeted the rebel-held town of Douma with shelling and air raids on Saturday.
By late evening, the Observatory said 20 civilians had been killed, but also that rescuers were still searching for more victims.
"Overnight, they found more victims underneath the rubble, and others who were wounded died," Abdel Rahman said on Sunday.
He said some strikes killed entire families inside their homes.
Local volunteers and civil defence teams worked on Sunday morning to locate seven people who were still unaccounted for, the Britain-based Observatory said.
The Douma Coordination Committee, a local activist group, published the names and photos of documented casualties on Facebook.
One picture showed a toddler the group said had been rescued from beneath the rubble but who succumbed to his wounds shortly afterwards.
At least 11 air strikes targeted the Eastern Ghouta area, of which Douma is a part, on Sunday. The Observatory said several people had been wounded, but had no further details.
On August 16, a series of regime strikes killed more than 117 people -- mostly civilians -- in Douma, sparking international condemnation of the Assad regime.
Eastern Ghouta has been under a government siege for nearly two years and comes under regular air attack.
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Meanwhile, northeast of Damascus at least 11 people were killed and 56 wounded by rocket fire outside Adra prison on Sunday, state television said.
It said the rockets were fired by "terrorist groups" and that women and children were among the wounded.
The Observatory's Abdel Rahman said nine people were killed, including civilians.
Syria's war, which began in March 2011 with anti-regime protests, has spiralled into a multi-front conflict that has killed more than 240,000 people.
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