At least 30 civilians, including women and children, were killed in Friday's air raids, the group said.
The intense bombardment could be meant to pressure the insurgent group to evacuate the city as many fear that the death toll could be high in an all-out battle to retake the town.
Those still trapped in Douma had been nervous that any attempt to renege on plans to leave would prompt Russian Federation and regime warplanes to resume deadly strikes.
In the province of Aleppo, a bomb went off in a rebel-held town on Saturday, killing at least eight civilians, local activists reported.
The Observatory said the Russian response to the Army of Islam with a list of demands including handing over heavy weapons within three days that should be followed by handing over light weapons and in return government forces will withdraw from the outskirts of Douma within a week.
Deadly air strikes slammed into the last opposition-held town in Syria's Eastern Ghouta on Friday for the first time in over a week, after talks sputtered over a rebel withdrawal.
Moscow stepped in to negotiate a deal for Douma, the third and final pocket where Jaish al-Islam had been angling for a reconciliation agreement that would allow them to stay as a police force.
"Jaish al-Islam terrorists are in a state of collapse and their media outlets are (making) chemical attack fabrications in an exposed and failed attempt to obstruct advances by the Syrian Arab army", state news agency SANA reported, citing an official source.
In an earlier report, the Observatory said the Islam Army was discussing a new deal with Russians and the Syrian side after suspending evacuation and release of kidnapped people.
Six civilians were killed and dozens injured as the rebels from Douma shelled the capital Damascus.
The government-controlled Syrian Central Military Media said government forces captured several farms Saturday on the southern and western edges of the city that is home to tens of thousands of people.
Douma residents, meanwhile, said they were living in limbo.
More than 1,600 civilians have been killed since then, according to the Observatory. The evacuations were suspended Thursday and state TV said Army of Islam members have refused to release scores of government supporters they have been holding for years.
Tens of thousands of civilians also fled through humanitarian corridors opened by Russian Federation and are either already returning to their devastated neighborhoods or staying in crowded shelters.
Syrian troops also carried out a ground operation Friday in the orchards surrounding Douma.
Leading Jaish al-Islam commander Mohammed Alloush told AFP that the Syrian regime were to blame for the collapse of the talks.
The latest fatalities bring to 48 the number of civilians who have been killed in the town since Friday when the Syrian military started a major campaign on the town shattering 10 days of calm there, the Britain-based watchdog said.
"The regime is trying to tighten the noose around Douma from the west, east, and south", said Observatory chief Rami Abdel Rahman.