An explosion in Syria's northwestern city of Idlib killed at least 23 people and injured tens more, including civilians, a war monitor reported on Sunday (Jan 7).
The Syrian Observatory for Human Rights said the explosion targeted the headquarters of a minor rebel faction in Idlib.
The nature of the attack was not immediately clear. The monitor said there were conflicting accounts, attributing the explosion that shook the Thalatheen district of the city to either a car bomb or a drone attack.
Ambulances were deployed to the explosion site and rescue teams were continuing efforts to recover bodies and the injured from the rubble of the targeted building and neighbouring houses, the monitor added.
The majority of the deaths were fighters from the rebel group, in addition to seven civilians, with several people still unaccounted for, it said.
"A large explosion on Sunday evening hit the base of the Ajnad al-Qawqaz faction in Idlib," said Syrian Observatory for Human Rights head Rami Abdel Rahman, adding that most of the non-civilian casualties were fighters from the group.
Dozens of people were wounded, particularly fighters, according to Abdel Rahman who said the base was "almost completely destroyed" and that nearby buildings were damaged.
The Ajnad al-Qawqaz group includes hundreds of Caucasian fighters and is battling alongside the Fateh al-Sham Front, a former Al-Qaeda affiliate, to repel a Syrian regime advance in the southeast of Idlib province.
Idlib province is a stronghold of rebels in Syria and is situated on the border with Turkey, one of the main backers of rebellion against President Bashar al-Assad.
The Syrian army and its allies launched an offensive in October to recapture the provinces of Idlib and Hama, and it has since been making swift advances.
The area has seen intense clashes following a regime offensive aimed at seizing a strategically vital highway between Damascus and second city Aleppo.
The main rebel force in Idlib is Tahrir al-Sham, spearheaded by the former al-Qaeda affiliate in Syria previously called Nusra Front.
The Syrian army lost Idlib to insurgents when the provincial capital fell in 2015. It became the only province fully under opposition control.
The Britain-based Observatory, which relies on a network of activists across Syria, said regime forces had seized more than 60 villages in the area since Dec 25.
An alliance dominated by Fateh al-Sham controls much of Idlib province where there are regular car bombings, often blamed on disputes between armed factions.
Some residents blame the Islamic State group for such attacks, although the group has no open presence in the province.