At least 22 air strikes pounded a key rebel bastion east of the Syrian capital on Wednesday after a local freeze on fighting expired overnight, a monitoring group said.

The Syrian Observatory for Human Rights said the suspected regime raids hit Eastern Ghouta as clashes with rebels erupted.

The fighting was centred on the town of Deir al-Assafir, where March air strikes by the regime killed 33 civilians, 12 of them children.

There was no immediate word on any casualties from the renewed fighting Wednesday, the Observatory said.

The municipal council in the flashpoint town of Douma confirmed that clashes and air strikes were rocking Eastern Ghouta, a belt of countryside and small towns east of the capital.

Moscow and Washington reached a deal last week on a temporary "freeze" in fighting in Eastern Ghouta and in the Mediterranean coastal province of Latakia.

The so-called "regime of silence" is meant to reinforce a broader truce brokered by the two world powers in February.

The halt in fighting was initially set for 24 hours in Eastern Ghouta but was extended twice, according to Syria's armed forces, and finally expired on Tuesday night.

A military source in Damascus confirmed to AFP on Wednesday that it was not renewed a third time.

The fresh fighting does not bode well for efforts to agree a "regime of silence" in the battleground second city of Aleppo, ravaged by nearly two weeks of surging violence.

More than 270,000 people have been killed since the conflict erupted in Syria in March 2011.