BEIRUT (AP) — A Syrian opposition spokesman and a war monitor say an airstrike on a training camp in the last rebel enclave in northwestern Syria has killed over 50 fighters.

Youssef Hammoud, a spokesman for the Turkey-backed Syrian opposition groups, said the airstrike Monday is believed to have been carried out by Russia. Hammoud said it targeted a training camp in Idlib province run by Failaq al-Sham, one of the largest rebel groups in the Turkey-backed opposition.

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.