The aircraft of regime ally Moscow targeted a military post of opposition fighters in the village of Madaya in the south of Idlib province, the Syrian Observatory for Human Rights said.

The Idlib region is supposed to be protected from a massive regime assault by a September buffer zone deal, but the area of three million people has come under increased regime and Russian fire since late April.

That region is administered by Syria's former Al-Qaeda affiliate Hayat Tahrir al-Sham, but other jihadist and opposition factions are also present.

Tuesday's strikes came after the United States on Monday said it had carried out a rare strike the previous day against Al-Qaeda-linked operatives, which the Observatory said killed six commanders of the Hurras al-Deen group in Aleppo province.

Fighting on the edges of the Idlib bastion also killed 15 regime forces and four jihadists and rebels on Tuesday, the monitoring group said.

Russian and regime bombardment have killed more than 500 civilians in the Idlib region since April 30, the Observatory says.

The war monitor says it relies on sources inside Syria for its information and determines who carried out air strikes according to flight patterns, ammunition and aircraft involved.

The buffer zone deal signed by Russia and rebel backer Turkey was never fully implemented as jihadists failed to retreat from the planned demilitarised area.

The civil war in Syria has killed more than 370,000 people and displaced millions since it started in 2011 with a deadly crackdown on anti-government protests.