The Britain-based Syrian Observatory for Human Rights could not say who carried out the strikes on the town of Zamalka in a southern pocket of the enclave.

Regime forces have retaken 70 percent of the last rebel bastion on the outskirts of the capital since February 18, carving it up into three shrinking pockets held by different rebels.

"Warplanes targeted civilians in Zamalka as they prepared to flee" the southern area of the enclave held by the Faylaq al-Rahman rebel group, Observatory head Rami Abdel Rahman said.

The regime assault has killed more than 1,390 civilians in the enclave, according to the Observatory, which relies on a network of sources on the ground.

The offensive has pushed thousands more to flee their homes into government-controlled areas.

On Saturday morning, "around 10,000 civilians streamed out of the rebel enclave into regime-held areas", Abdel Rahman said.

More than 40,000 civilians have poured out of the enclave since Thursday morning, fleeing bombardment and advancing troops.

Syria's war has killed more than 350,000 people and displaced millions since it broke out in 2011 with the brutal repression of anti-government protests.