Turkish-backed militia Faylaq al-Sham blames Russia for an airstrike on its training camp in northwestern Syria which it claimed had killed more than 50 of its fighters and promised Tuesday to retaliate.

The daytime airstrike dealt one of the heaviest blows to the rebel group - destroying one of its most elite training facilities. Syrian opposition representative, Youssef Hammoud, said that the airstrike happened in the north-west Idlib province, which is one of the last remaining territories held by the rebel armed group.

The rebel training camp located in Jebel al-Dweila trained new recruits for the Faylaq al-Sham. Sources claimed that leaders of the camps along with fighters in training were among those that were killed by the attack.

The rebel group published a post following the attack, stating that Russia will need to answer for its "crime." The group's representative, Naji al-Mustafa, told reporters that the National Front for Liberation will respond to the "violations," indirectly threatening Russian and Syrian government positions.  

Journalists were barred from going near the area. The actual extent of the damage is not immediately clear. Both Turkey and Russia have yet to make a formal comment on the incident.

A report published by the UK-based Syrian Observatory for Human Rights claimed that authorities may have played down the damage caused by the alleged Russian airstrike. In its estimate, the organization said that at least 78 fighters were likely killed by the attack and around 90 were wounded. The organization, which is closely monitoring the war in Syria, also concluded that the attack was carried out by Russia. Putin is known to be a close ally of Syrian President Bashar al-Assad.

A doctor in a hospital in Idlib city located near the site told reporters that they were overwhelmed with injured following the strike. The hospital was reportedly forced to send some of the wounded and the dead to other facilities. The doctor, who had chosen to remain anonymous, said that that all of the injured were fighters.

Earlier in the year, Russia had brokered a truce between Turkey and Syria to end the attacks that had displaced hundreds of thousands of residents. The truce had remained shaky as minor skirmishes continued to happen now and then. Last week, the Syrian Observatory for Human Rights reported that an airstrike had hit a local fuel market in opposition-controlled Jarablus in northern Aleppo.