At least 40 people were killed Friday when Syrian government forces launched missiles into a crowded, rebel-held neighborhood near the capital, according to war monitors.
The Syrian Observatory for Human Rights said about 100 people also were wounded in the attack on Douma, just northeast of Damascus. The death toll is likely to rise, it added.
The Syrian government has not commented on the strikes, which the Observatory said were continuing as of midday Friday.
Opposition fighters have regularly traded fire with the Syrian military in Douma, which lies within a major rebel stronghold.
Government strikes on Douma have repeatedly wounded or killed large numbers of civilians. An August attack on a market there killed at least 112 people, mostly civilians.
The Syrian conflict has killed an estimated 250,000 people since it began in 2011.