Syrian opposition forces killed at least 15 Moscow-backed Bashar Assad regime fighters in retaliation for Russian strikes that left dozens dead among their ranks in northwestern Idlib province, a war monitor said Tuesday.

Moscow's airstrikes Monday hit a training camp of the Syrian National Army’s (SNA) Faylaq al-Sham faction near the Turkish border in Syria's last major opposition bastion Idlib, killing 78 fighters and wounding 90 more.

The National Liberation Front (NLF), a group of the opposition based in Idlib that includes Faylaq al-Sham, vowed retaliation.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said 15 pro-regime 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 told Agence France-Presse (AFP) "the NLF immediately responded" by targeting regime positions, especially in the south of Idlib province and north of Hama province.

Monday's strikes were the bloodiest surge in violence since a Russian-Turkish truce came into force almost eight months ago in northwestern Syria.

That 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.

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.