DAMASCUS (AFP) - The United Nations was poised to deliver aid to desperate civilians besieged in rebel-held areas of Syria's third city Homs on Saturday on the second day of a humanitarian truce.

The planned relief convoys come after 83 children, women and elderly people who had survived more than 600 days under tight army blockade were evacuated from the war-battered enclave on Friday.

The Homs evacuation and aid delivery was made possible by a surprise UN-brokered deal between the government and rebel commanders on the ground to observe a three-day "humanitarian pause" in hostilities, which largely held on Friday, UN officials said.

The long-sought truce had eluded mediators in last month's fruitless first round of peace talks between government and opposition delegations in Switzerland which are due to resume in Geneva on Monday.

The desperately needed food and medicines have been held up for months in a UN warehouse in a government-controlled area just kilometres away from the trapped civilians awaiting the ceasefire required for their safe delivery.

"Aid convoy is now being loaded and prepared to go to the Old City of Homs," the Red Crescent tweeted shortly before 11 am (0900 GMT).

UN officials said the trucks would carry emergency rations for 2,500 people food, medical kits and bedding, as well as cash and other support for the "immediate needs both of those who choose to be evacuated from the area and of those who remain inside".

Even after Friday's evacuation of the first batch of civilians who chose to leave, hundreds more women, children and elderly remain among the 2,500 plus residents still inside.

Activists say they have been surviving on little but olives and wild cereals for months.

UN spokesman Farhan Haq said there had been sporadic shooting during the evacuation of civilians on Friday but that both sides broadly observed the ceasefire.

"We understand that for the most part the operation went smoothly, but there were isolated reports of gunfire heard during the day," he said.

However activists accused government forces Saturday of firing mortar rounds on rebel-held districts in violation of the truce.

"Besieged neighbourhoods, particularly those close to Hamadiyeh where the aid convoys are due to enter, have been targeted by mortar fire from pro-government areas," an activist in the Old City, who gave his name only as Yazan, said.

The exiled opposition expressed concern that the promised aid delivery could be aborted, saying it would be "devastating to those innocent civilians who remain within the area under siege".

"We call on the world to look carefully at what happens in Homs in the coming hours and days."