An air strike on an opposition training camp in northwestern Syria on Monday killed more than 50 fighters and wounded nearly as many, a Syrian opposition spokesman and a war monitor said.
The air strike in the northwestern part of Idlib province, the last opposition enclave in Syria, targeted a military training camp for Failaq al-Sham, one of the largest Turkey-backed opposition groups in Syria, said Youssef Hammoud, a spokesman for the groups.
The Britain-based Syrian Observatory for Human Rights, which monitors the war in Syria, said the strike killed 56 fighters and wounded nearly 50.
Rescue missions are 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 war.
Leaders of the camp were among those killed in airstrike in Jabal al-Dweila, according to Hammoud.
Turkey and Russia had brokered a truce in Idlib earlier this year to halt a government offensive that displaced hundreds of thousands. But the truce remained shaky.
Turkey has long supported Syrian opposition forces in Syria. Russia has negotiated with Ankara to deploy observation teams in the enclave to monitor the truce.