Idlib and parts of the neighbouring provinces of Aleppo, Hama and Latakia are under the control of Hayat Tahrir al-Sham, a jihadist group led by Syria's former Al-Qaeda affiliate.

The region is supposed to be protected from a massive government offensive by a September buffer zone deal, but it has come under increasing bombardment by the regime and its Russian ally over the past three months.

On Saturday, two children were among 11 civilians killed in air raids on the Idlib town of Ariha, said the Syrian Observatory for Human Rights.

The attack wounded 28 others, said the Britain-based monitor.

Two residential buildings in Ariha were hit by raids, in the second such attack on the town this week, said Observatory head Rami Abdul Rahman.

Regime air strikes killed 10 civilians in Ariha on Wednesday, according to the Observatory.

Bombardment by government forces on other parts of the Idlib region killed another civilian on Saturday and wounded 15 others, the monitor said.

Air strikes by the Syrian regime and its ally Russia on the Idlib region have claimed more than 740 lives since late April, according to the Observatory.

The UN says more than 400,000 people have been displaced.

The UN Office for the Coordination of Humanitarian Affairs (OCHA) has documented 39 attacks against health facilities or medical workers in the area in three months.

At least 50 schools have been damaged by air strikes and shelling over the same period, it said.

"These are civilian objects, and it seems highly unlikely, given the persistent pattern of such attacks, that they are all being hit by accident," UN rights chief Michelle Bachelet said on Friday.

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