Russia says Syria's Government has withdrawn from a key road leading into the city of Aleppo, but senior UN officials say the Syrians are holding up aid deliveries to areas where civilian have been living under siege for years.

The retreat comes following urges from the United Nations to "immediately" allow humanitarian aid into the country after a fragile ceasefire was extended for 48 hours by Russia and the United States.

Under the deal brokered by Russia and the US, all forces were supposed to withdraw from the Castello Road, which leads into the city and would become a conduit with special status.

Senior UN officials have gone public with the frustration over the Syrian government's go-slow — holding up aid deliveries to areas where civilian have been living under siege for years.

Jan Egeland, special advisor to the UN's top envoy on Syria, said the government of Bashar al Assad was still standing in the way of aid reaching areas his army has under siege.

"Can well-fed grown men please stop putting political, bureaucratic and procedural road blocks for brave humanitarian workers that are willing to go to serve women, children, wounded civilians in besieged and crossfire areas?

"We are ready to go, but the facilitation letters, the permits have not been given yet. Not a single permit is in hand," Mr Egeland said.

While UN agencies have been accused of acquiescing too readily to government restrictions, Mr Egeland said the bureaucracy of the permits has a simple, and brutal logic to it.

"In some cases we wouldn't be able to load, because the security forces would be there when we load the truck, in other cases, the first road block, the armed [forces] would block our drivers from reaching women and children."

The UN's Special Envoy on Syria Staffan de Mistura said even Russia, which backs the government, is frustrated by the go-slow.

"Are we disappointed? Of course we are. But who is particularly disappointed are the Syrian people."

UN officials have expressed frustration with the Syrian Government holding up aid deliveries to areas under siege for years. ( AFP )

Forty aid trucks carrying food supplies for 80,000 people had been waiting on the border with Turkey to get into the rebel held east of Aleppo where an estimated 250,000 people are living under siege by government forces.

But the UN had been unable to get the necessary security guarantees from all sides.

In the past, government forces have been notorious for stopping aid convoys into other besieged areas and taking out vital supplies like medicine, and the UN has been criticised for going along with it.

But Mr de Mistura said that on the Castello road at least, that was not allowed under the ceasefire deal.

The United States said on Thursday that a ceasefire which began on Monday was more or less holding, despite breaches by both sides.

ABC/AFP