At least 30 civilians, including women and children, were killed in Friday's air raids, the group said.
Backed by Russia, Syrian troops had captured all of the one-time opposition stronghold of Ghouta - except for Douma, its largest town which is held by the Jaish al-Islam Islamist faction.
The regime and its ally Russian Federation launched a blistering air and ground offensive on Eastern Ghouta in mid-February, killing more than 1,600 civilians and causing an worldwide outcry.
Fighters and civilians with familial ties to the group are being rejected, residents said.An activist interviewed Friday said the choice to stay had not been his own, after Jaish al-Islam declined to clear two of his relatives to leave and held up his application as a result.
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Syria and Russian Federation had threatened a resumption of their brutal assault if Jaish al-Islam did not agree to the deal.
Rebels in Eastern Ghouta have managed to keep Syrian military forces at bay during years of war, but a four-year government siege of the district has led to a humanitarian crisis with severe shortages of food and medicine. The group wants to remain in Douma and none of the fighters have left, according to Mohammad Alloush, a representative for Jaish al-Islam. "People still need me", he said.
Reports suggest the Jaish al-Islam rebels remaining in the city are increasingly split on the question of evacuating.
For a week the bombardment had subsided as Moscow pursued peace talks with Jaish al-Islam, but talks came to an abrupt halt and air strikes suddenly resumed on Friday afternoon, killing dozens of civilians.
It remains unclear exactly why the talks fell apart this week.
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A total of 2,963 militants and their families evacuated Douma in three batches over the past few days under a deal concluded between the Islam Army and the Syrian government forces under a Russian mediation. "There were 20 strikes in 15 minutes", Mohammed, a medic, told AFP.
"The talks were going well", he said, but power struggles between the regimes allies had caused them to break down.
The wounded were being brought in with fractures and other injuries, and there had been three amputations on Friday alone, he said.
Syrian troops were matching their renewed bombing with a ground operation in the orchards surrounding Douma.
The Britain-based Observatory said warplanes were hitting across Douma today, as regime artillery fire hit neighbouring agricultural fields.
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"The regime is trying to tighten the noose around Douma from the west, east, and south", said Observatory chief Rami Abdel Rahman.