Six children were among 12 people killed on Tuesday in government fire on several towns in Idlib province and neighbouring Aleppo countryside, the Syrian Observatory for Human Rights said.

A hospital in the Idlib town of Kafranbel was hit by artillery shells, said David Swanson, a spokesman for the UN humanitarian office.

"The facility is reportedly out of service due to severe structural damage," he told AFP.

The hospital's administrative director Majed al-Akraa confirmed the attack.

"The hospital is completely out of service," he said.

"It was a strong attack. The generators and even my car caught fire," he told AFP.

It follows two days of intensified regime bombardment on the region that killed a total of 31 civilians on Sunday and Monday, according to the Britain-based Observatory.

Rescue volunteers and civilians were seen pulling dust-covered victims from the rubble of destroyed buildings in the wake of those strikes.

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 the jihadist bastion has come under increasing bombardment by the regime and its ally Russia since late April.

The Observatory says over 260 civilians have been killed in the spike in violence since then.

More than 200,000 civilians have already been displaced by the upsurge of violence, according to the United Nations.

The UN has warned an all-out offensive on the region would lead to a humanitarian catastrophe for its nearly three million residents.

At least 20 health facilities have been hit by the escalation -- 19 of which remain out of service, the UN has said.