Parts of Homs Old City have been under army siege since June 2012. Photo: BBC

The ceasefire should also allow long-awaited aid into the worst hit areas.

The UN welcomed reports that a "humanitarian pause" had been agreed with Syrian authorities. Rebels are also expected to back the deal.

Parts of Homs Old City have been under army siege since June 2012 and many areas lie in ruins.

US State Department spokeswoman Jen Psaki welcomed the agreement but said an evacuation was "not a substitute for the safe, regular and unfettered delivery of humanitarian assistance".

"We should not be giving credit to a regime just for providing food for a few days to people who are starving, given that's the right moral thing to do," she said.

"This is something they should have been doing all along."

The situation in besieged districts of Homs was discussed during peace talks in Geneva a week ago.

Up to 3,000 civilians are believed to be trapped by the fighting and activists say people have survived on little more than olives for weeks.

Farhan Haq, spokesman for UN Secretary General Ban Ki-moon, said supplies were positioned on the outskirts of Homs "ready for immediate delivery as soon as the green light was given by the parties for safe passage".

Many families in Homs have lost everything in the fighting

Aid staff were also on standby, he said.

The Syrian foreign ministry said that under the deal - reached between the governor of Homs and the UN resident co-ordinator in Syria - "innocent civilians" would be allowed out of besieged areas.

"We are very happy that finally we found the possibility to bring out these people and to provide those who are needy inside old Homs with humanitarian aid they deserve," Deputy Foreign Minister Faisal al-Mekdad said.

"The only precondition is that this aid and the help should not go to terrorists or armed groups."

Although rebels have not made any statement, a BBC correspondent in the region says the Syrian Red Crescent has received positive signals that the deal will go ahead.

The Red Crescent told the BBC the hope was for an evacuation to take place on Friday and for supplies to be taken in to remaining civilians the following day.

The BBC's Jim Muir in neighbouring Lebanon says it has taken many days of detailed negotiations for the deal to be reached.

In the past, similar plans for an evacuation of civilians have gone off the rails at the last minute.

Homs - Syria's third largest city - has been a key battleground in the uprising against President Bashar al-Assad.

Much of the city initially fell under rebel fighters' control, but government forces have since retaken many areas, forcing the opposition into the Old City.