At least 20 civilians were reportedly killed after air strikes hit Syria's rebel-held eastern Ghouta district, just outside Damascus.
The British-based Syrian Observatory for Human Rights claimed that jets also struck the towns of Zamalka, Arbaeen, Hazza and Beitu Soua.
"Dozens of air strikes hit several areas in Eastern Ghouta, killing 23 civilians including four children," said Rami Abdel Rahman, head of the observatory.
The deadliest raids reportedly hit a market in the town of Beit Sawa, killing nine civilians, including two children.
Another six civilians, including a young girl, were killed in bombardment on the town of Hazzeh.
The Syrian army of President Bashar al-Assad, backed by Russian air power and Shia Muslim militias, has besieged eastern Ghouta for years.
The area is the largest remaining rebel bastion near Damascus.
International concern has been growing over the fate of eastern Ghouta as bombardment by the Syrian military and Russia has increased in recent months and as shortages of food and medicine have led to severe malnutrition.
This month alone, chlorine is suspected of having been used on two occasions in munitions dropped on eastern Ghouta, causing respiratory problems among civilians.
Last Friday, a Syrian regime bombardment killed at least 11 civilians.
Mr Assad and his allies have gained momentum in the seven-year war, gaining territory, pushing rebels from most big cities, and forcing several pockets to surrender last year.
Syria's war has killed more than 340,000 people and displaced millions since it began in March 2011 with the brutal repression of anti-government protests.
The US and Russia clashed at the UN Security Council this evening over the use of chemical weapons in Syria.
Washington accused Moscow of shielding Mr Assad from accountability, while Russia accused the US of slander.
