Thousands of Eastern Ghouta residents are being evacuated from the besieged enclave on Sunday, Syrian state media reported, after large groups of residents already left towns in the Damascus suburb over the past days.

State news agency Sana reported that buses carrying about 500 residents and fighters were about to leave the Irbin corridor, with hundreds more expected to leave later on Sunday.

Two out of three rebel groups that had been controlling the enclave since 2013 have already surrendered, with thousands of fighters and their relatives departing for opposition-held areas in Idlib, a province in the country's north.

On Saturday, opposition fighters and their families were evacuated as part of an evacuation deal that was reached between the Russian army and two main rebel groups in the area.

More than 900 residents of Eastern Ghouta started leaving towards Idlib province on Saturday night, an activist inside the city told Al Jazeera.

Residents have also been leaving from Harasta, which was previously controlled by Ahrar al-Sham rebel group, on board 17 buses, and from Irbin, which along with Zamalka and Jobar had been controlled by the Faylaq ar-Rahman rebel group.

According to SANA, 25 buses have been sent to the outskirts of Irbin to transport residents from towns and surrounding areas towards opposition-held territory in the country's north.

Laith al-Abdullah, an activist in Harasta, told Al Jazeera that this would be the second and "last" evacuation - as per the agreement struck with the Russians. "It is unlikely there will be a third evacuation ... but we will wait and see," he said.

The spokesperson of Failaq al-Rahman, Wael Olwan, criticised the United Nations for not offering residents who wish to remain in Eastern Ghouta protection or guarantees.

"The first people to leave from areas in Ghouta will be the wounded, and the last to leave will be the fighters," Olwan told Al Jazeera.

On February 18, Syrian government forces, backed by Russian fighter jets, tightened their siege on Eastern Ghouta with a heavy military offensive that killed 1,500 and wounded more than 5,000.

Nearly 400,000 people remained in the enclave before the latest offensive began.

Last week, state media reported some 4,000 people left Harasta in the Damascus suburb, including more than 1,000 Ahrar al-Sham opposition fighters, after the rebel group reached an evacuation deal on Wednesday.

Douma rebels hold on

Meanwhile, fighters from Jaish al-Islam in Douma, the largest rebel-held town remaining in Eastern Ghouta, have not been able to reach a deal with the Russian army - the Syrian forces' main ally - with about 150,000 residents still trapped inside.

The people remaining in Douma face limited access to food and medicine, activists told Al Jazeera.

Those slated for transport to Idlib will likely face a new set of challenges there, as Syrian military and Russian air raids on Idlib have increased in the past week, killing dozens of people.

The province has also seen sustained fighting between rebel groups.