They belonged to Hurras al-Deen, which maintains ties to the global jihadist network and fights alongside Al-Qaeda's former Syria branch, Hayat Tahrir al-Sham.

HTS dominates most of Idlib province as well as parts of neighbouring Hama, Aleppo and Latakia.

"Two leaders were killed, one Tunisian and another Algerian, in a missile attack that targeted a meeting... in the western countryside of Aleppo," the Syrian Observatory for Human Rights said.

Fourteen other fighters and a commander were wounded in the attack, it said, adding that the source and type of strike was unknown.

The greater Idlib area was supposed to be protected by a buffer zone under an September agreement between Russia and Turkey.

But backed by its ally Moscow, Damascus has since late April ramped up its bombardment of the region, home to some three million people -- nearly half of whom have been displaced from other parts of Syria.

That came after HTS seized most of Idlib at the start of the year.

Since its formation in 2014, the US-led coalition against the Islamic State group has also targeted jihadist leaders in Idlib, but the strikes have dropped off significantly since 2017.

Hurras al-Deen was established in February 2018 and has some 1,800 fighters, including non-Syrians, according to the Observatory.