"A large explosion this 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.
He did not specify the cause of the blast, but activists on social media said a car bomb was responsible.
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.
The area has seen intense clashes following a regime offensive aimed at seizing a strategically vital highway between Damascus and second city Aleppo.
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 December 25.
Some residents blame the Islamic State group for such attacks, although the group has no open presence in the province.