A week-old truce in Syria brokered by the US and Russia appears close to unravelling with alleged violations by government and rebel forces mounting.
A US-backed rebel group in the divided northern city of Aleppo said the initiative had “practically failed”.
Rebels also warned that the forced evacuation of some 300 fighters from a besieged suburb of the central city of Homs would effectively end the truce.
Government warplanes meanwhile bombed rebel positions in Hama province.
State media reported that the strikes had killed dozens of fighters from the jihadist group Jabhat Fateh al-Sham, which was known as al-Nusra Front until it broke off formal ties with al-Qaeda in July.
The Syrian military had also yet to announce an extension of its seven-day “regime of calm”, which it said expired at midnight.
On Sunday, the Syrian air force reportedly targeted rebel-held areas of Aleppo for the first time since Monday and dropped barrel bombs on a town in the south.
The BBC’s James Longman in Beirut says there were never high hopes for the cessation of hostilities, but after just one week it looks to be in serious trouble.
“I believe that practically it has failed and has ended,” Zakaria Malahifji, head of the political office of the Aleppo-based group Fastaqim, told the Reuters news agency on Monday morning.
Asked whether he expected aid to be delivered to the 250,000 people living in the city’s besieged rebel-held east – a key part of the truce deal – Mr Malahifji said: “There is no hope. It has been a number of days of procrastination. Every day there is a pretext.”
The UN’s aid chief, Stephen O’Brien, said on Monday that he was “pained and disappointed” that a 20-lorry aid convoy destined for eastern Aleppo was still stuck on the border with Turkey because it had not received necessary permissions and safety guarantees from the Syrian government.
A number of leading rebel factions also warned that if the government pressed ahead with its plan to evacuate more rebel fighters from the besieged Homs suburb of al-Wair, it would “have clearly ended its commitment to any proposed truce”.