At least 23 people, most of them from the Ahrar al-Sham militant group, were killed by a truck suicide bomber in Syria's Aleppo city on Monday, a monitor said.

The opposing Britain-based Syrian Observatory for Human Rights said 19 fighters from the group and four civilians were killed in the blast in the Sukari district of the city.

The attack hit an Ahrar al-Sham checkpoint at the entrance to a security complex belonging to the group.

The Observatory said several prisoners being held by Ahrar al-Sham at a facility in the complex were also believed to have been killed, but it had no immediate toll.

The blast destroyed three buildings, and people were still missing under the rubble, the monitor added.

It was unclear who was responsible for the blast, but Ahrar al-Sham has been targeted by assassinations in the past.

In September 2014, most of its top leadership was wiped out in a devastating explosion at its headquarters in Idleb province that killed nearly 50 people.