The civilians were killed in regime air strikes and shelling on the south of Idlib province and the north of Hama province Thursday, the Syrian Observatory for Human Rights said.

Russian and regime air strikes also killed 21 jihadist and Islamist fighters in the same region the same day, the Britain-based monitor added.

The Idlib region of some three million people is supposed to be protected from a massive regime offensive by a buffer zone deal that Russia and Turkey signed in September.

But it was never fully implemented, as jihadists refused to withdraw from a planned demilitarised zone.

In January, the Hayat Tahrir al-Sham alliance led by Syria's former Al-Qaeda affiliate extended its administrative control over the region, which includes most of Idlib province as well as adjacent slivers of Latakia, Hama and Aleppo provinces.

The Syrian government and Russia have upped their bombardment of the region since late April, killing more than 360 civilians, according to the Observatory.

Syria's war has killed more than 370,000 people and displaced millions since it started in 2011 with the repression of anti-government protests.

Russia launched a military intervention in support of the regime in 2015, helping its forces reclaim large parts of the country from opposition fighters and jihadists.