Air strikes and mortar attacks continued on the besieged Syrian enclave of Eastern Ghouta on Tuesday, despite a Russian-organised ceasefire.

The truce broke down minutes after its scheduled 9am start, with government troops shelling the rebel pocket before opposition fighters returned fire.

The violence prevented the delivery of aid and left the humanitarian corridor impassable.

"We have reports this morning there is continuous fighting in Eastern Ghouta," said Jens Laerke, UN humanitarian spokesman. "Clearly the situation on the ground is not such that convoys can go in or medical evacuations can go out."

The UN Security Council last week unanimously passed a resolution demanding a 30-day cessation of hostilities. However, Russia, which backs the Syrian government, unilaterally proposed a less ambitious plan for a five-hour daily "humanitarian pause" to allow in aid and open up safe corridors for civilians to evacuate.