Britain-based Syrian Observatory for Human Rights says unidentified aircraft carried out the strike near the town of Sarmada.
An air strike in Syria on Tuesday killed at least 25 members of Jabhat Fateh al-Sham, formerly known as the Nusra Front, including senior figures, a war monitor said.
Unidentified aircraft "hit a major base of Fateh al-Sham near the town of Sarmada" in the northwestern province of Idlib, Syrian Observatory for Human Rights director Rami Abdel Rahman said.
Tuesday was the fifth day of a ceasefire in Syria which its sponsors, Russia and Turkey, say excludes Daesh and the Syrian militia Jabhat Fateh al-Sham.
Abu Anas al-Shami, a Jabhat Fateh al-Sham spokesman, said that the attack was carried out by the international coalition. More than 20 people were killed, he added.
"The headquarters targeted by the international coalition a short time ago are a main headquarters for that area and contain a number of branch offices, leading to the killing of the brothers," al-Shami said.
Jabhat Fateh al-Sham changed its name in July 2016 and announced it was severing ties with Al Qaeda.
The Observatory said eight Jabhat Fateh al-Sham fighters and leaders have been killed in air strikes around rebel-held Idlib in January.
The latest strikes come as several rebel groups say they are pulling out of peace talks scheduled for later in January in Kazakhstan.
The rebels released a joint statement expressing their frustration with ongoing violations of the ceasefire by Syrian regime forces. They say they won't take part in the negotiations.
Earlier TRT World spoke to the chief negotiator for Syrian opposition forces, Usama Abu Zeid. He explained what the rebel groups want.