At least 23 people, mostly from the Syrian rebel group Ahrar al-Sham, were killed by a suicide bomber in a tanker truck in Aleppo on January 25, a monitoring group said.
The Britain-based Syrian Observatory for Human Rights said 19 fighters from the powerful Islamist group, including four commanders, and four civilians were killed in the blast in the Sukari district of the city.
The attack hit an Al-Sham checkpoint at the entrance to a security complex belonging to the group.
Several prisoners being held at a facility were also believed to have been killed in the blast, which destroyed three buildings, the monitor said.
It was unclear who was responsible for the blast, but Al-Sham has been targeted by a string of assassinations recently.
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.
Suspicion has fallen on the Syrian regime and the Islamic State, the conservative religious group’s rival.
The observatory said an Al-Sham commander and 11 family members also were killed on January 25 in a Russian air strike, with three children among the dead.
Al-Sham is a leading member of the Army of Conquest alliance that controls Idlib Province along with Al-Qaeda affiliate Al-Nusra Front.