A US-backed rebel group in the divided northern city of Aleppo said the initiative had “practically failed”.
He said US and Russian officials were meeting in Geneva on Monday to discuss developments.
Earlier, Russia’s defence ministry said rebel violations had made it “pointless” for Syrian forces to uphold the truce.
“Considering that the conditions of the ceasefire are not being respected by the rebels, we consider it pointless for the Syrian government forces to respect it unilaterally”, Lt Gen Sergei Rudskoi said in a televised statement.
Syria 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.
On Sunday, the Syrian air force reportedly targeted rebel-held areas of Aleppo for the first time since last 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”.
A pro-government Lebanese TV channel reported that the rebels and their families had left on Monday, but a local official later told Reuters that the evacuation had been postponed until Tuesday.
The truce was dealt another blow on Saturday when warplanes from the US-led coalition against so-called Islamic State (IS) accidentally bombed Syrian troops in the eastern city of Deir al-Zour.
President Bashar al-Assad called the strikes, which officials said killed more than 60 soldiers, the “latest example of flagrant American aggression against Syrian army positions in the interests of the terrorist organisation Daesh [IS]”.
On Monday, the Ministry of Defence in London confirmed that the British aircraft – believed to be unmanned, remotely-piloted Reaper drones – had been involved in the strike, along with jets from Australia and Denmark
A statement said the UK was “fully co-operating with the coalition investigation” and stressed that it “would not intentionally target Syrian military units”.
Meanwhile, Turkish-backed Syrian rebel forces might extend their zone of control in northern Syria and seek to capture the IS stronghold of al-Bab, Turkish President Recep Tayyip Erdogan has said.
Mr Erdogan told a news conference in Istanbul that 900 sq km (350 sq miles) of IS territory had been cleared since last month, and that he envisaged the “safe zone” eventually extending up to 5,000 sq km.