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, a monitor said.

The 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 addition, a Russian air strike killed 12 people, including a local commander of the group, in the northwestern province of Idlib, the monitoring group said. It was not immediately clear where the warplane struck. The Observatory said it targeted the commander's house, but rebel sources said it struck an office of the group.

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.

And more recently, the group has seen several of its commanders 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.

Ahrar al-Sham is one of Syria's most powerful rebel groups, and is a leading member of the Army of Conquest alliance that controls Idlib province along with Al-Qaeda affiliate Al-Nusra Front.