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.

Four civilians -- a woman and three children -- were killed in an air strike in the town of the Saraqib, the Syrian Observatory for Human Rights said.

An AFP reporter at the scene saw rescue workers extract the body of a woman from the rubble of a destroyed building.

After night fell, they searched for other victims, the lamps on their white helmets casting light onto the ruins in the dark.

Earlier, two children were killed in a garage on the edge of the town of Maaret al-Numan, while another child died in the village of Maar Zita, the Britain-based monitoring group said.

Two other civilians perished in strikes on the villages of Kansafra and Khan al-Subul, it said.

The September deal signed by Russia and rebel backer Turkey was supposed to set up a buffer zone around the Idlib region, but it was never fully implemented as the 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.

Since late April, more than 460 civilians have been killed in government or Russian bombardments, according to the Observatory.

The violence has forced around 330,000 people to flee their homes and hit 23 health centres, the United Nations says, warning of one of the worst humanitarian crises of the eight-year civil war.

The conflict in Syria 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.