Air strikes on a building in northern Syria used by an insurgent group formerly known as the Nusra Front had killed at least 25 people and wounded dozens more on Tuesday, the Syrian Observatory for Human Rights said.
The British-based Observatory said it was unclear whether the strikes had been carried out by aircraft belonging to Russia or to the US-led coalition.
Tuesday is the fifth day of a ceasefire in Syria but its sponsors, Russia and Turkey, say this excludes Islamic State and the Syrian Islamist militia Jabhat Fateh al-Sham, formerly called the Nusra Front.
Abu Anas al-Shami, a Jabhat Fateh al-Sham spokesman, said the attack, in the countryside of the northern province of Idlib, was carried out by the international coalition. More than 20 people were killed, he said.
"The headquarters targeted by the international coalition a short time ago are a main headquarters for that area and contains a number branch offices, leading to the killing of the brothers," he said in a statement.
Jabhat Fateh al-Sham changed its name in July 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.