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 killed five civilians, including two sisters under the age of 10, in the village of Josef in Idlib province, said the Syrian Observatory for Human Rights.

The strikes also wounded several people, some of whom were in serious condition, said the Observatory.

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 died in regime and Russian bombardment there.

In addition, 780 rebel and jihadist fighters and more than 610 pro-regime fighters have died in raids and clashes on the ground.

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.