Syrian opposition activists are reporting the first airstrikes on rebel-held neighbourhoods in the northern city of Aleppo since a U.S. and Russian-brokered truce went into effect.

The Britain-based Syrian Observatory for Human Rights says warplanes fired four missiles on Sunday. The Syrian military had previously said it would adhere to the week-long ceasefire until midnight Sunday.

The Local Coordination Committees, another monitoring group, says Sunday's air raids targeted three neighbourhoods in eastern Aleppo. The LCC and the Observatory say a number of people were wounded in the air raids.

Moscow said militants there were preparing for large-scale military actions against Syria's army.

'The truce...will not hold out'

Insurgents say they only reluctantly accepted the initial deal to relieve the dire humanitarian situation in besieged areas they control, and blamed Russia for undermining the truce.

"The truce... will not hold out," a senior rebel official in Aleppo said.

Rebels have also accused Russia of using the ceasefire to give the Syrian army and allied Shia militias a chance to regroup and deploy forces for their own offensives.

Both sides have reported dozens of violations since the truce went into effect Monday evening.