A senior member of the Syrian Ahrar al-Sham rebel group was killed along with six other fighters on Tuesday in a double suicide bombing in northwestern Syria, a monitor said.
No group claimed responsibility for the attack, but the Syrian Observatory for Human Rights said it was believed to have been carried out by a group linked to ISIS.
The Britain-based monitor said seven members of Ahrar al-Sham, a conservative Islamist rebel group, were killed in the blast near the town of Salqin.
Among them was Abu Abdel Rahman Salqin, described by Observatory chief Rami Abdel Rahman as "one of Ahrar al-Sham's most senior leaders."
Ahrar al-Sham is one of the most powerful rebel groups in northern Syria and belongs to an alliance with Al Waeda affiliate Al Nusra Front that has seized most of Idlib province in recent months.
Despite its Islamist ideology, Ahrar al-Sham is opposed to ISIS.
In September 2014, 47 members of Ahrar al-Sham's leadership were killed when a blast hit a meeting of its top religious and military chiefs in Idlib.
No group claimed responsibility for that bombing, which forced the group to quickly establish a new leadership, but it was also attributed to ISIS.
Ahrar al-Sham is one of the oldest and largest of Syria's armed opposition groups, established in 2011 by Islamists released by the Syrian regime early in the uprising against President Bashar al-Assad.
Over the weekend, the group's foreign relations head wrote an opinion piece published in The Washington Post, criticizing US policies towards Syria.
Labib Al-Nahhas accused Washington of too narrowly defining the term "moderate" and said Ahrar al-Sham had been "unfairly vilified."
More than 230,000 people have been killed in Syria since the conflict began in March 2011.
AFP contributed to this report.