Syrian rebel group Jaish al-Islam released five prisoners including civilians on Wednesday as part of a deal over the eastern Ghouta town of Douma, the Syrian Observatory for Human Rights said.

The UK-based war monitor said the insurgents had kidnapped them from the nearby town of Adra around five years ago.

Syrian state TV separately said the army had freed four women and a man from the militants as part of "efforts...to liberate all the prisoners in Douma."

Syrian government forces, backed by Russia, have now recaptured nearly all of eastern Ghouta, on the outskirts of Damascus, from the rebels in a ferocious assault that began in February, marking a major victory for President Bashar al-Assad.

Jaish al-Islam holds more than 3,500 prisoners and hostages in total, Rami Abdulrahman, director of the Observatory, told Reuters.

State media has said Jaish al-Islam had accepted a deal giving its fighters safe passage to towns that are located in a buffer zone on the border with Turkey that is controlled by the Turkish military and allied Syrian rebel groups.

Jaish al-Islam has not confirmed any deal with the Syrian government over eastern Ghouta. On Tuesday an opposition source said talks were continuing with Damascus, even as state media reported the departure of another group of fighters from eastern Ghouta.

The state news agency SANA said on Wednesday buses had entered Douma for the third consecutive day to evacuate more rebel fighters and their families to the northern Syrian city of Jarablus, near the border with Turkey.

The group, which is estimated to have many thousands of fighters, has previously insisted it will not leave Douma or accept "forced displacement" to another part of Syria.

A military source told Reuters on Monday that some elements of Jaish al-Islam were still rejecting a deal and that military force would be used if they refused to accept one.

The source said on Tuesday the government had set a deadline for militants to leave Douma, without saying how long.