Syrian government airstrikes on rebel-held Eastern Ghouta have killed at least 31 people including 12 children, according to a UK-based monitoring group.
The bombing also injured around 60 others.
The Syrian government has repeatedly said it only targets militants.
For three days running the besieged area east of the Syrian capital has been bombed by warplanes, causing buildings to collapse on residents.
The United Nations has described the situation in Ghouta as "extreme". It follows a call by its Humanitarian Affairs office for a month-long ceasefire throughout Syria to allow aid deliveries.
With a Syrian government offensive against rebel-held Idlib and Turkish-led fighting in the Kurdish enclave of Afrin, the UN is warning of people in danger on multiple fronts.
In a separate development more than 100 fighters aligned with Syrian President Bashar al-Assad were killed after U.S.-led coalition and coalition-backed local forces thwarted a large, apparently coordinated attack late on Wednesday and early on Thursday, a U.S. official said.
No American troops were killed or wounded in the incident, officials said. although some U.S. troops had been embedded at the time with Syrian Democratic Forces (SDF), whose headquarters in Syria's Deir al-Zor province had been a target of the attack.
The heavy death toll on the Syrian government side underscored the large size of the attack, which the U.S. official said included about 500 opposing forces, backed by artillery, tanks, multiple-launch rocket systems and mortars. The official spoke to Reuters on condition of anonymity.
One member of the Russian parliament said what he called an American attack on forces in Syria was an act of agression against Russia.