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

The Syrian Observatory for Human Rights said 19 fighters from the rebel 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 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 Idlib province that killed nearly 50 people.

Several of its commanders were among a string of Islamist rebels killed in mysterious targeted killings.

The attacks have not been claimed, and suspicion has fallen on either the regime or the Islamic State group, which considers all forces that have not pledged allegiance to it to be rivals.

Some have suggested that a rivarly between Ahrar al-Sham and Syria's al-Qaeda affiliate could also be connected.

The attack comes hours after another devastating attack on the rebel group.

An Ahrar al-Sham commander and 11 members of his family were killed in a Russian air raid on village between northern Idlib and western Aleppo province.

Three children were among the dead, but the monitor gave no further details about the commander.

Ahrar al-Sham is one of Syria's most powerful rebel groups and a leading component of Idlib's Jaish al-Fatah alliance.