Russia said the Syrian army had begun to withdraw from a road into Aleppo on Thursday, a prerequisite for pressing ahead with international peacemaking efforts, as the government and rebels accused each other of violating a truce.

An organisation that monitors the war also said the Syrian army had begun moving away, but insurgent groups in Aleppo said they had not seen the army withdrawing from the Castello Road, needed to allow aid deliveries into the city, and would not pull back from their own positions near the road until they did.

The Pentagon said it could not confirm reports of a withdrawal but US State Department spokesman Mark Toner said the ceasefire was holding "by and large", adding both Washington and Moscow believed it was worth continuing.

But there were growing accusations of violations by each side, with a Syrian military source saying the rebels were responsible for dozens of breaches including gun, rocket and mortar fire in Damascus, Aleppo, Hama, Homs and Latakia. The rebels said Syrian army jets had struck in Hama and Idlib, and used artillery near Damascus.

The Syrian Observatory for Human Rights, a British-based war monitor, said it had documented attacks by both sides, and that despite widespread calm between rebels and the army, the first civilians had been killed since the truce began on Monday.

Two civilians killed on Thursday were children in government-held areas, one in Aleppo and the other in Syria's southwest, it said. In addition, air strikes against Islamic State militants in the town of al-Mayadin near Deir al-Zor had killed at least 23 civilians, it said.

Control of the Castello Road is divided between the government and rebels who have been battling to topple President Bashar al-Assad for more than five years. It has been a major front line in the war.

"The Syrian army...began the staged withdrawal of vehicles and personnel from the Castello Road to ensure the unimpeded delivery of aid to eastern Aleppo," said Lieutenant-General Vladimir Savchenko, head of the Russian Reconciliation Centre in Syria in remarks broadcast on state television.

The Observatory said the army had started to withdraw from positions on the road, but that Russian troops, whose air force has helped Damascus blockade rebel-held Aleppo, had replaced it.

An official in an Aleppo-based Syrian rebel group said late on Thursday the army had not pulled back. "There is no withdrawal by the regime from the Castello Road," Zakaria Malahifji, of the Aleppo-based rebel group Fastaqim, told Reuters.

UN waits for permits

The UN Syria envoy Staffan de Mistura said the United States and Russia were expected to manage the disengagement of forces from the road, but criticised Damascus for failing to provide permits needed to make aid deliveries to other areas.

The UN humanitarian adviser Jan Egeland said both the rebels and the government were responsible for delaying aid deliveries into Aleppo.

"The reason we're not in eastern Aleppo has again been a combination of very difficult and detailed discussions around security monitoring and passage of roadblocks, which is both opposition and government," he said.

In other areas, de Mistura was categorical about blaming the Syrian government, saying it had not yet provided the proper permits. The Syrian government has said all aid deliveries must be conducted in coordination with it.

France, which backs the opposition, became the first US ally to publicly question the deal with Moscow, urging Washington to share details of the agreement and saying without aid for Aleppo, it was not credible.

About 300,000 people are thought to be living in eastern Aleppo, while more than one million live in the government-controlled western half of the city.

Two convoys of aid for Aleppo have been waiting in no-man's land to proceed to Aleppo after crossing the Turkish border.

If a green light was given, a spokesman for the UN Office for the Coordination of Humanitarian Affairs said the first 20 trucks would move to Aleppo and if they reached the city safely, the second convoy would then leave. The two convoys were carrying enough food for 80,000 people for a month, he said.

The United States and Russia have backed opposing sides in the Syrian war that has killed hundreds of thousands of people, forced 11 million from their homes, and created the world's worst refugee crisis since World War 2.

Aleppo, Syria's biggest city before the war, has been a focal point of the conflict this year. Government forces backed by militias from Iran, Iraq and Lebanon have recently achieved their long-held objective of encircling the rebel-held east.

Moscow criticises Washington

Russia's intervention a year ago in support of Assad has given it critical leverage over the diplomatic process.

Its ally, Assad, appears as uncompromising as ever. He vowed again on Monday to win back the entire country, which has been splintered into areas controlled by the state, an array of rebel factions, the Islamic State group, and the Kurdish YPG militia.

Washington hopes the pact will pave the way to a resumption of political talks. But a similar agreement unraveled earlier this year, and this one also faces enormous challenges.

Russia's deputy foreign minister Gannady Gatilov said talks could resume at the end of September, but this was rejected by George Sabra, an opposition negotiator, who said conditions on the ground were not yet good enough.

The United States and Russia are due to start coordinating military strikes against Islamic State and a group formerly known as the Nusra Front if all goes to plan under the deal.

But Russia said on Thursday the United States was using "a verbal smokescreen" to hide its reluctance to fulfill its part of the agreement, including separating what it called moderate opposition units from terrorist groups.

The defence ministry said only government forces were observing the truce and opposition units "controlled by the US" had stepped up shelling of civilian residential areas.

Rebels say Damascus has carried out numerous violations.

While the general lines of the agreement have been made public, other parts have yet to be revealed, raising concerns among US allies such as France, which is part of the coalition attacking Islamic State in Syria and Iraq.

The State Department said it was not ready to publish the pact but would discuss it in detail with partners on the sidelines of next week's UN General Assembly meetings in New York.

Separately, a US official said its detailed provisions include establishing two checkpoints on the Castello Road to be operated by the Syrian Red Crescent and allowing all Syrians to leave Aleppo on the road, including opposition forces with weapons.

The official, who spoke on condition of anonymity, was confirming details first reported by the Associated Press.