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 24 kilometers ( 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.

“We, the factions of the National Front for Liberation, will respond to these violations,” said Naji al-Mustafa, another spokesman for the Turkish-backed fighters, threatening to target government and Russian positions. He called the strike a “crime” by Russia.

Turkey and Russia brokered a truce in Idlib earlier this year to halt a government offensive that displaced hundreds of thousands in the already overcrowded enclave. Around a dozen Turkish observation points were deployed inside Idlib to monitor the truce, which remained shaky.

In recent days, there was a resumption of strikes.

On Friday, airstrikes also targeted a local market for rudimentary fuel burners and diesel in the opposition-controlled region of Jarablus, in northern Aleppo. At least seven people were killed, according to the Observatory.

Last week, Turkish troops evacuated one of their largest military bases in the area, which was surrounded by Syrian government troops for months. Syrian opposition fighters said it was part of Turkey's redeployment of its forces in the shrinking enclave.