The Britain-based monitor said it could not confirm whether the strikes were carried out by Syrian government warplanes or those of its ally, Russia.
Strikes began again on the besieged opposition enclave on Friday with at least 40 civilians killed in a day of shelling and air strikes on Douma.
State TV showed thick clouds of smoke rising from the targeted area, the town of Douma, where the Jaish al-Islam rebel group is holding out after insurgents in other parts of eastern Ghouta accepted safe passage to other rebel areas.
Backed by Russia, Syrian troops have recaptured 95 percent of Ghouta since February 18 through a combination of a deadly air and ground assault and evacuation deals.
A steady trickle of buses had been carrying residents from Douma for nearly a week, apparently under the terms of an agreement struck between Jaish al-Islam and Russian Federation.
State news agency SANA said the suspension was the result of disagreements within the Army of Islam rebel group, adding that buses that entered Douma for the evacuations on Thursday returned without passengers.
"The regime is trying to tighten the noose around Douma from the west, east, and south", said Observatory head Rami Abdel Rahman.
"The negotiations failed and the regime wants its conditions - the air strikes are a taste of what could happen if its conditions are not implemented", he said.
Top Jaish al-Islam political figure Mohammad Alloush on Friday said the talks had been going "well" until a power struggle emerged between the regime's allies.
Syrian opposition activists and state media are reporting airstrikes and shelling in and around the rebel-held town of Douma after days of calm, signaling a possible collapse of a truce and evacuation deal.
SANA blamed the suspension of the evacuation on the inner conflict among Islam Army rebels. A ground assault then sliced the area into three isolated pockets, each held by different rebel factions.
Several thousand people left Douma in recent days, taken by bus to the rebel-held northern town of Jarablus, which is next to the Turkish border.
But the exit plan stalled after reports that Jaish Al Islam remained divided over a withdrawal, and heavy bombing started to hit Douma on Friday afternoon.
Those still trapped in Douma had been nervous recently that any attempt to renege on an evacuation would only prompt Russian Federation and the Syrian aviation to resume deadly strikes.
Jaish al-Islam said its rocket and artillery brigade were responding to what it described as a massacre by "the Assad militias and their ally the Russian warplanes".