Al-Qaeda-linked group accused of trying to take advantage of partial ceasefire in Syria to exert its influence.
Air strikes on two markets in Syria have killed more than 50 people, local sources told Al Jazeera, as opposition negotiators said that they were leaving peace talks held in Geneva.
The strikes in the northwest of the country were among the worst since a ceasefire was called at the end of February, Rami Abdulrahman, chief of the UK-based Syrian Observatory for Human Rights, said.
The opposition Higher Negotiations Committee (HNC) bloc announced that its delegation was leaving Geneva because no advances were made in the talks.
Riad Hijab, the head of the HNC, had earlier warned that the delegation would leave Switzerland if attacks on rebel-held towns would not stop.
It was not clear if Tuesday’s air strikes were carried out by Syrian or Russian planes that have been deployed to Syria to support President Bashar al-Assad.
Rescue workers said more than 40 people were killed in the town of Maarat al-Numan in the rebel stronghold of Idlib province.
Another 10 people were killed in an air strike on a market in the nearby town of Kafr Nubl. The deaths from the latest strike included five children, sources told Al Jazeera.
“We have more than 20 cars that have been moving dead and injured to hospitals in the area,” said Ahmad Sheikho, a member of the civil defence corps, a rescue service operating in opposition-held territory.
“The air strike at the central vegetable market in [Maarat al-Numan] was around noon. Another strike at the same time hit Kafr Nubl’s own market,” Sheikho said.
The observatory, which monitors the conflict through a network of sources on the ground, gave a slightly lower toll, saying at least 37 people were killed in Maarat al-Numan and seven were killed in Kafr Nubl.
Many of the injured were in a serious condition and the death toll was expected to rise, the observatory said.
The Syrian military could not immediately be reached for comment, and state news agency SANA made no mention of air strikes in Idlib.