Pro-Turkish Syrian rebels pounded Moscow-backed regime forces with rocket and artillery fire in northwest Syria Tuesday, a day after Russian aircraft killed dozens among their ranks, a monitor said.

The 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 an 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 abroad in Libya and in the Caucasus over Nagorno-Karabakh.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said the NLF bombarded pro-government fighters in regime-held areas adjacent to the stronghold.

"The factions of the National Liberation Front have since Monday evening fired hundreds of rockets and artillery shells on areas controlled by regime forces in the south and east of Idlib," as well as parts of the adjacent Hama, Aleppo and Latakia provinces, the Observatory said.

NLF spokesman Naji Mustafa told AFP: "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.

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 the 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.

It was not immediately clear if there were casualties. 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, who are 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.