Syrian rebels have killed at least 15 Moscow-backed Syrian regime fighters in retaliation for Russian strikes that left dozens dead among their ranks, a war monitor said Tuesday.

Moscow's airstrikes Monday hit a training camp of the Faylaq al-Sham faction near the Turkish border in Syria's last major rebel bastion of Idlib, killing 78 fighters and wounding 90 more. The National Liberation Front (NLF), an umbrella group of Ankara-backed rebels based in Idlib that includes Faylaq al-Sham, vowed retaliation.

Faylaq al-Sham is a Sunni Islamist group that has acted as Turkey's proxy during several Turkish military campaigns on Syrian soil and has also been the source of pro-Ankara mercenaries sent to fight in Libya and in the Caucasus over Nagorno-Karabakh.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said 15 pro-government fighters were killed over the past 24 hours in NLF bombardment on regime-held areas in the south and east of Idlib, as well as parts of the adjacent Hama, Aleppo and Latakia provinces.

NLF spokesman Naji Mustafa said: "The NLF immediately responded" by targeting regime positions, especially in the south of Idlib province and north of Hama province. "The retaliation is ongoing and will be fierce," he said.

Monday's strikes were the bloodiest surge in violence since a Russian-Turkish truce came into force almost eight months ago in northwest Syria. The deal stemmed a Russia-backed regime offensive on the bastion that had killed more than 500 civilians and displaced almost a million people in one of the worst humanitarian crises of Syria's nine-year civil war.

NLF spokesman Mustafa accused the Russians of scuppering the months-long ceasefire with Monday's airstrikes. Also on Tuesday, fighters loyal to Damascus fired back on rebels in the south of Idlib and Hama province, the Observatory said.  At least one rebel fighter was killed on the edges of the town of Saraqeb in Idlib, according to the war monitor. The last major rebel stronghold covers around half of Idlib province as well as slivers of adjacent provinces.

It is dominated by Hayat Tahrir al-Sham, a coalition led by Syria's former Al-Qaeda affiliate, but other rebels including the NLF are also present. Syria's war, which broke out after the brutal repression of anti-government protests in 2011, has killed more than 380,000 people and displaced millions at home and abroad.