BEIRUT: Syrian regime air strikes killed four civilians including two children Sunday in a northwestern bastion of opposition, a Britain-based war monitor said.

The Idlib region of some three million people is supposed to be protected by a September buffer zone deal, but the jihadist-run enclave has come under mounting bombardment by the government and its ally Russia since late April.

An air raid by Damascus aircraft on Sunday took the lives of two adults and two children in the village of Josef in Idlib province, the Syrian Observatory for Human Rights said.

Since Wednesday, more than 60 civilians have been killed in regime air raids in the surrounding anti-government enclave, the Britain-based monitoring group says.

Since the start of the escalation in late April, more than 470 civilians have perished in regime and Russian bombardment there.

The violence has forced around 330,000 people to flee their homes and hit 23 health centres, the United Nations says.

The September deal signed by Russia and rebel backer Turkey was supposed to set up a buffer zone around the Idlib region, but the accord was never fully implemented as jihadists refused to pull back from the front lines.

In January, Hayat Tahrir al-Sham -- an alliance led by Syria's former Al-Qaeda affiliate -- took over administrative control of the region, which is made up of a large part of the Idlib province and slivers of neighbouring governorates.

The Syria conflict has killed more than 370,000 people and displaced millions since it erupted in 2011 with the brutal repression of protests against President Bashar al-Assad.