The remaining rebels in Syria's battered Eastern Ghouta have surrendered their heavy weapons and their leader has left the enclave for the north, a war monitor said on Thursday.

Syria's regime is poised to secure the opposition's onetime bastion on the edge of Damascus, after a negotiated withdrawal for its last holdout of Douma.

"Jaish al-Islam fighters handed over their heavy weapons to Russian military police in the town of Douma on Wednesday," said the Syrian Observatory for Human Rights.

"Most of Jaish al-Islam's top brass, including their head Issam Buwaydani, left Douma and arrived in northern Syria on Wednesday evening," the Britain-based monitor said.

His departure came under a fraught deal for Douma announced last weekend, just hours after an alleged chemical weapons attack that killed dozens in the town.

Under the agreement, thousands of Jaish al-Islam fighters and civilians have been bussed out of Douma to opposition-controlled parts of Syria's Aleppo province.

It was unclear whether more evacuations would take place Thursday.

Russian military police began patrolling Douma on Thursday as part of the deal, according to the Russian defence ministry.

It also said the Syrian government flag was flying on Thursday in the town, which signalled that the regime was back in control of Ghouta although its forces have not yet entered Douma.

Ghouta was the opposition's main bastion on the edge of the capital and suffered a five-year regime siege before the ferocious assault that Moscow and Damascus began in February.