The Syrian Observatory for Human Rights said on Sunday that rebel group Failaq al-Sham has started withdrawing its forces and heavy arms from a demilitarised zone in northwest Syria.
The group is the first to comply with a requirement to leave a demilitarised buffer zone set up by Turkey and Russia to avert a Russian-backed Syrian army offensive, Rami Abdulrahman, head of the UK-based war monitor told Reuters.
Rebel sources could not immediately be reached for comment.
The group is withdrawing its forces and heavy arms in small batches from southern Aleppo countryside, adjacent to Idlib province, which is part of the DMZ towards the west
The demilitarized zone will be 15 to 20 km (10 to 12 miles) deep, run along the contact line between rebel and government fighters, and will be patrolled by Turkish and Russian forces.
Turkey and Russia had agreed in mid-September to enforce a new demilitarized zone in Idlib province from which rebels will be required to withdraw by the middle of next month.
Failaq al-Sham is the third largest group among the rebel groups in Northwest Syria, according to the monitor.
The biggest jihadist group, Tahrir al-Sham, has yet to announce its position regarding the agreement.