The Syrian Observatory for Human Rights described as a "massacre" the strikes on a vegetable market in Maaret al-Numan that killed at least 37 civilians and at least another seven at a fish market in nearby Kafranbel.

Previously, the Britain-based monitor had put the toll in Maaret al-Numan at five dead and dozens wounded.

The two locations, both in Idlib province, are only about 10 kilometres apart.

Both under the control of Al-Qaeda's Syrian offshoot Al-Nusra Front, which like the jihadist Islamic State group is excluded from a ceasefire agreed between mainstream rebels and the regime of President Bashar al-Assad.

Elsewhere in the province, the Observatory said that three children were killed in shelling by Islamist rebels on the government-held Shia village of Kafraya. 

In Damascus province, at least seven people were killed and 10 wounded in strikes on Bala, in the rebel-stronghold region of Eastern Ghouta, the monitor added. 

And fighting continued between rebels and regime forces in Latakia province, an Assad stronghold, after an alliance of opposition armed groups launched a joint offensive over the regime's truce "violations".

A ceasefire deal agreed between the United States and Russia, which came into effect in February, excludes areas controlled by Al-Nusra and the Islamic State group. 

The truce however has been strained by increased fighting around second city Aleppo, and in other areas of Syria, where both the regime and its opponents have accused each other of a string of breaches. 

Syria's main opposition group said Monday it was suspending its official participation in UN-brokered peace talks in Geneva over what it said was the government's repeated "violations" of the truce.