BEIRUT (AP) — An airstrike on a rebel training camp in northwestern Syria on Monday killed more than 50 Turkish-backed fighters and wounded nearly as many, in one of the heaviest blows to the opposition’s strongest groups, a spokesman and a war monitor said.

The opposition vowed to retaliate for the attack on Faylaq al-Sham, blaming Russia for the daytime airstrike. There was no immediate comment from Russia or Turkey, which although they support opposite sides in Syria’s conflict, have worked together to maintain a cease-fire in the rebel enclave.

Youssef Hammoud, a spokesman for the Syrian opposition, said the airstrike in the northwestern part of Idlib province, the last rebel enclave in Syria, targeted a military training camp for Faylaq al-Sham. Faylaq al-Sham is the largest Turkey-backed armed group and one of the most disciplined and best trained.

Turkey has long supported Syrian rebel forces in Syria and has used many of those fighters to bolster its military campaigns in Libya and Azerbaijan.

The camp, at Jebel al-Dweila not far from the Turkish border, was hosting training sessions for new recruits, according to a war monitor and another opposition spokesman. Leaders of the camp were among those killed, according to Hammoud.

Journalists or activists in the area were not allowed near the camp and the extent of the damage was not immediately known,

The Britain-based Syrian Observatory for Human Rights, which monitors the war in Syria, gave a higher toll, at 78 fighters dead and nearly 90 wounded. Rescue efforts were still underway, the Observatory said. It said it also suspected the airstrike was carried out by Russia, which is a close ally of Syrian President Bashar Assad in the country’s civil war.

A hospital near the explosion was overwhelmed with the casualties and was forced to send wounded and dead to other facilities. A doctor in Idlib city said the city’s central hospital, more than 15 miles from the camp, received two bodies and 11 wounded. All the casualties were fighters, the doctor said, speaking on condition of anonymity because he was not allowed to discuss the attack on an armed group. One media activist, Rashid al-Bakr, was among those killed, according to the Macro Media Center, an online news platform.

One Facebook group called on Idlib residents to check with hospitals in the city if they are missing relatives, a clear indication many remained unidentified.

Syrian rebel groups vowed to retaliate.