BEIRUT: The UN's top aid official in Syria said an evacuation of civilians was expected on Tuesday out of the eastern Ghouta rebel enclave, where the military has waged a fierce offensive.

The army's campaign of air and artillery strikes has killed more than 1,100 civilians in almost a month, the UN Office for the Coordination of Humanitarian Affairs says.

Syrian government forces have captured more than half the enclave, splintering it as they seek to crush the last major rebel bastion near the capital Damascus. Their advances have cut off Douma and Harasta from each other and from other neighbouring towns.

Families are sleeping in the open in the streets in Douma, Ghouta's biggest town, with no more room in basements to shelter from the bombs, local authorities have said.

"This day, we are expecting evacuation of civilians, including people with medical conditions," the UN's resident humanitarian coordinator in Syria, Ali al-Za'tari, told Reuters at al-Wafideen crossing near Douma, without elaborating.

The United Nations has previously called for the evacuation of nearly 1,000 sick and wounded people who need urgent medical treatment.

Jaish al-Islam, one of the main rebel factions there, said on Monday it had reached an agreement with the government's key ally, Russia, to evacuate wounded people.

Still, Hamza Birqdar, the faction's military spokesman, vowed on Tuesday that the fighters would defend the Ghouta till the end.