A suicide car bomb hit the headquarters of Ahrar al Sham in Idlib province, according to the Syrian Observatory of Human Rights (SOHR).
Activists say the car bomb in Armanaz town killed at least three and wounded more than 15 others.
The car bomb came amid some of the heaviest fighting between two rival groups, Ahrar al Sham and Hayat Tahrir al Sham.
Fighting on Wednesday spread to areas across Idlib, including the town of Saraqeb in the east, Dana and Sarmada in the northeast and Bab al Hawa near the Turkish border, SOHR reported.
Hayat Tahrir al Sham is thought to be behind the suicide attack.
The group is mainly made up of Syria's former Al Qaeda affiliates known as Nusra Front.
Ahrar al Sham on the other hand has Turkish-backed Free Syrian Army (FSA) members in its ranks.
Tensions on the rise
The two groups have been engaged in skirmishes since the beginning of the year as they differ over ideologies.
Hayat Tahrir al Sham are a more hardline pro-Wahhabi outfit while Ahrar al Sham are a more moderate group.
Idlib residents took to the streets on Tuesday and raised the Syrian opposition flag that represents the Syrian revolution.
Hayat Tahrir al Sham fighters opened fire at a young man who had raised a revolution flag.
He and two other people were wounded, including a child.
Overnight, Tahrir al Sham fighters replaced the revolution flags with black ones and clashes broke out again with Ahrar al Sham in several towns.
As a result, Tahrir al Sham seized control of a town called Hazarin.
Two civilians were killed in the fighting and several were wounded.