A week-old Syrian ceasefire brokered by the US and Russia was in deep trouble on Monday as a rebel official said it had practically failed and signalled insurgents were preparing for a full resumption of fighting.

Already widely violated since it took effect, the ceasefire came under added strain at the weekend when Russia said jets from the US-led coalition against Islamic State killed more than 60 Syrian soldiers in eastern Syria.

Syrian President Bashar Al-Assad called the incident "flagrant aggression". Washington has called it a mistake.

The agreement is the second ceasefire negotiated by Washington and Moscow this year in the hope of advancing a political end to a war now in its sixth year, which has killed hundreds of thousands of people.

But while it has led to a significant reduction in fighting over the past week, violence has been increasing in recent days. A planned delivery of humanitarian aid to besieged rebel-held districts of eastern Aleppo - one of the first steps in the deal - has been repeatedly postponed.

Plans to evacuate several hundred rebels from the last opposition-held district of Homs city have also overshadowed the agreement, with rebels saying it would amount to the government declaring the ceasefire over. The Homs governor said the plan had been postponed from Monday to Tuesday.

US Secretary of State John Kerry said the ceasefire was "holding but fragile". If the truce were to collapse, it could doom any chance of President Barack Obama's administration negotiating a Syria breakthrough before it leaves office in January.

Kerry overcame scepticism of other administration officials to hammer out the ceasefire, gambling on cooperation with Russia despite the deepest mistrust in decades between the Cold War-era superpower foes. Washington and Moscow back opposite sides in the war between Assad's government insurgents, while both oppose the Islamic State jihadist group.

The politburo chief of one prominent Aleppo rebel group, Fastaqim, said the agreement had "practically failed and has ended", adding that it remained to be seen if anything could be done "in theory" to save it.

Zakaria Malahifji, speaking to Reuters from the Turkish city of Gaziantep, also indicated rebel groups were preparing for combat: "I imagine in the near future there will be action by the factions".

Another rebel official also signalled the insurgents might soon step up military action.

Abu Al-Baraa Al-Hamawi, commander of a group fighting in the Jaish Al-Fatah Islamist alliance, said it was time for a new attempt "to break the siege on thousands of civilians in Aleppo after the false promises of aid deliveries from the United Nations".

Monitors reported clashes in and around Aleppo on Monday. The government blamed some of the violence on an what it said was an insurgent assault, but another rebel official denied they had yet launched new attacks.

The opposition High Negotiations Committee spokesman Riad Nassan Agha said the government side had never committed to the truce.

"Air raids by Russian and Syrian warplanes, which haven't stopped, suggest the truce never started in the first place," he said.

The Syrian army meanwhile had yet to announce any extension of the seven-day ceasefire it declared on September 12, which was due to expire at 11:59 pm (2059 GMT) on Sunday, according to the statement issued by the army command when the truce was announced.

The US-Russian deal set out steps including a nationwide ceasefire, aid deliveries, and joint US-Russian targeting of jihadists including Islamic State and a faction formerly affiliated to Al Qaeda.

Washington hopes it will lead to talks on ending a war that has splintered Syria, uprooted 11 million people and created the world's worst refugee crisis.

But the ceasefire deal has faced enormous challenges from the outset, including how to disentangle nationalist rebels backed by the West from jihadists not covered by the truce.

And there has been no sign of compromise on the issue at the heart of the war: the future of Assad, who enjoys firm Iranian and Russian military backing that is buttressing his strongest military position in years. The dispute over his fate has made a mockery of all previous diplomatic efforts to end the fighting.

The last ceasefire, reached in February, unravelled over a period of weeks as fighting intensified, particularly in and around Aleppo, Syria's biggest city before the war and now potentially the biggest prize for pro-government forces.

The UN aid chief said an aid convoy destined for rebel-held eastern Aleppo was still stuck in Turkey.

"I am pained and disappointed that a United Nations convoy has yet to cross into Syria from Turkey, and safely reach eastern Aleppo," the UN Under Secretary General for Humanitarian Affairs Stephen O'Brien said in a statement.

The United Nations says it still lacks sufficient security guarantees from both sides to deliver aid to eastern Aleppo, the rebel-held half of the city, which pro-government forces completely encircled this month.

Up to 275,000 people remain trapped in eastern Aleppo without food, water, proper shelter or medical care, he added.

UN officials have blamed Damascus for blocking aid deliveries to other besieged, rebel-held areas. - Reuters