Syrian regime warplanes carried out several air strikes in the Eastern Ghouta area east of Damascus on Sunday, a day after Bashar al Assad's forces declared a cessation of hostilities in the area, the Syrian Observatory for Human Rights said.
The British-based monitoring group said Saturday had been relatively calm after the ceasefire took effect with isolated incidents of shellfire.
On Sunday, six air strikes hit the towns of Douma and Ain Terma in opposition-held Eastern Ghouta, it reported.
There was no immediate comment from the regime or its forces.
The regime forces declared a "cessation of fighting activities" starting at noon on Saturday in besieged Eastern Ghouta, which has long been controlled by the opposition.
But no rebel group yielding influence in Eastern Ghouta said they had signed that agreement apart from one opposition group in Eastern Ghouta quickly welcomed the ceasefire.
A separate statement from Cairo-based political opposition movement Al Ghad, headed by Ahmad Jarba, said the agreement had been reached in Cairo, sponsored by Egypt and Russia and with the involvement of mainstream opposition groups.
There was to be a full ceasefire in Eastern Ghouta, no regime forces would enter the area and aid would be allowed in, it said.
The opposition enclave targeted in the strike today, is in one of four proposed "de-escalation zones" designated in a deal reached by regime allies Iran and Russia and rebel backer Turkey in May.
But the accord has yet to be fully implemented over disagreements on policing the safe zones.
A ceasefire was implemented in another "de-escalation zone" in southern Syria on July 9, but none has so far been announced for the northwestern province of Idlib or parts of the central province of Homs.
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More than 400,000 people have been killed in Syria since its conflict broke out in March 2011 with anti-regime protests.
Numerous attempts at a lasting ceasefire in western Syria, where opposition groups have lost ground to regime forces and their allies over the last year, have often collapsed with both sides trading the blame.
The United States, Russia and Jordan reached a ceasefire and "de-escalation agreement" for southwestern Syria this month, which has reduced violence. That agreement did not include Eastern Ghouta.