"At least thirteen Syrian and non-Syrian fighters" from the Hayat Tahrir al-Sham jihadist group were killed, said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights (OSDH).

Several other fighters were wounded in the strikes, with some in critical condition, said the Britain-based monitor, which relies on a network of sources inside Syria.

The monitoring group had earlier given a toll of eight dead.

Moscow's intervention since 2015 has helped Damascus claw back much of the territory it lost to rebel forces early in the 12-year civil war, and Russian forces have repeatedly struck the Idlib area.

Early Monday, "Russian warplanes carried out air strikes on the western outskirts of Idlib city, targeting a military base belonging to Hayat Tahrir al-Sham (HTS)," Rahman said.

Jihadist group HTS, led by Syria's former Al-Qaeda affiliate, controls swathes of Idlib province as well as parts of the adjacent Latakia, Hama and Aleppo provinces.

An AFP correspondent said the jihadist group cordoned off the area after the strikes, which came shortly after midnight (2100 GMT on Sunday).

HTS regularly carries out deadly attacks on soldiers and pro-government forces.

- Downed drones -

On Monday, the Syrian defence ministry said its forces had downed "three drones laden with explosives" operated by "terrorist organisations".

The Observatory said the army shot down three reconnaissance drones in Idlib and Hama provinces.

Syria's war broke out in 2011 after the repression of peaceful demonstrations by the government of President Bashar al-Assad escalated into a deadly conflict that pulled in foreign powers and jihadists insurgents.

Russian and Iranian support have helped to turn the tide in the regime's favour.

The war has killed more than half a million people and forced around half of Syria's pre-war population from their homes.

On August 5, three family members, all civilians, were killed when Russian warplanes struck the outskirts of Idlib city, the Observatory said at the time.

On June 25, Russian air strikes killed at least 13 people including nine civilians in Idlib province, in what the Observatory said was the deadliest such attack on the country this year.

A member of the Turkistan Islamic Party, a Uyghur-dominated jihadist group, was among the four fighters killed in those strikes, which also wounded at least 30 civilians, the monitor had said.

The rebel-held Idlib region is home to about three million people, around half of them displaced from other parts of the country.

Since 2020, a ceasefire deal brokered by Damascus's ally Russia and rebel-backer Turkey has largely held in Syria's northwest, despite periodic clashes.