Heavy airstrikes by Syrian government planes and their Russian allies on rebel-held areas in the southern and eastern countryside of Idlib have killed at least 43 people, mostly civilians, a monitoring group and activists said on Monday.

At least 37 people were killed when Russian warplanes bombarded a public market in Syria’s north-western province of Idlib, the Britain-based Syrian Observatory for Human Rights said.

It added that 105 people were also wounded in the strike in the city of Maaret al-Numann, and that more people were still under the rubble.

Syrian government planes also later hit the city of Saraqeb, killing at least six people, the observatory said.

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“This is the largest massacre to take place since the offensive started in April,” Rami Abdel Rahman, head of the observatory, told dpa.

Since late April, the forces of Syrian President Bashar al-Assad, supported by Russian air power, have been waging a massive campaign against rebels in the provinces of Hama and Idlib, the last major opposition strongholds in the country.

The offensive has so far killed 2,636 people, including 744 civilians, and displaced more than 300,000 people, according to the observatory