A fragile ceasefire to end the violence in Syria was in jeopardy on Sunday as Russia accused the United States of "criminal negligence" following an airstrike that killed dozens of Syrian government soldiers.

Meanwhile unidentified aircraft carried out airstrikes on Aleppo on Sunday, in the first attack on the city since the Russian and American brokered ceasefire brought combat operations to a halt last Monday.

The Syrian Observatory for Human Rights said several people were wounded but could not immediately give details about casualties or identify who carried out the strikes.

The airstrikes came as the Russian defence ministry warned that “tensions are rising” around Aleppo, the besieged city at the centre of a struggle between Bashar Assad’s government and opposition groups, and accused the United States of failing to deliver on a promise to disengage opposition and from “terrorist” forces.

The ministry said it suspected armed groups in the city of preparing a "mass offensive" under the cover of the ceasefire.

The statement came a day after Russia and Syria said aircraft from the US-led coalition struck Syrian government forces near city of Deir Ezzor, killing dozens of regime troops.

Russia said 62 soldiers were killed in the strikes, while the UK-based Syrian Observatory for Human Rights said the attack left at least 90 dead.