The Britain-based monitor said it could not confirm whether the strikes were carried out by Syrian government warplanes or those of its ally, Russia.

It was the second day of heavy bombardment on Douma, with at least 40 civilians killed on Friday in a sudden resumption of strikes after a more than week-long lull.

The renewed raids appear to be an attempt to pressure Jaish al-Islam -- the Islamist group that holds the town -- to agree to the regime's terms of a withdrawal.

Six civilians were also killed and dozens more wounded as Douma rebels shelled the capital Damascus on Saturday, Syrian state media said.

State television broadcast live footage from a hospital in Damascus, where pools of blood stained the floor and wounded could be heard wailing in pain.