Dozens of hostages released by rebels in Syria's Douma arrived at Syrian army lines on Monday night, state media said, under a Russian-brokered deal for the fighters to surrender the besieged town.

Rebels and civilians had started leaving the last opposition-controlled town in Eastern Ghouta on Sunday.

State news agency SANA said buses carrying Jaish al-Islam rebels and their families had exited the town of Douma and would head north to opposition-held territory.

The evacuations came after accusations Syria's government had used toxic gas on Douma, which it denied.

There was no immediate comment from Jaish al-Islam.

The rebel group has more than 3,500 prisoners and hostages in its prisons in Douma, Rami Abdul Rahman, the director of the UK-based Syrian Observatory for Human Rights monitoring group, told Reuters.

Five prisoners were released on Wednesday, after earlier departures by Jaish al-Islam fighters.

Russian news agency RIA, citing a security source, said Jaish al-Islam rebels will leave Douma in two batches in the coming hours.

Syrian President Bashar al-Assad has regained control of nearly all of Eastern Ghouta in a Russian-backed military campaign that began in February, leaving just Douma in rebel hands.

Tens of thousands of people are estimated to be sheltering in the battered city.

After a lull of a few days, government forces began bombarding Douma again on Friday.