The death toll resulting from a series of airstrikes in north-western Syria on Sunday has risen to 41, a monitoring group said a day after the attacks.

The death toll in the rebel-held city of Idlib has risen to 34, including nine children, the Britain-based Syrian Observatory for Human Rights said.

The group called the attacks "a massacre," saying unidentified warplanes targeted a marketplace and other locations of the city.

The dead include a mother, her five children and a young man, in the town of Maaret al-Nuaman, near Idlib.

Sunday's bombardments were the latest in a spate of aerial attacks on the mostly rebel-held Idlib province.

Last month, at least 30 people were killed in airstrikes on tent settlements sheltering refugees in a rebel-held area in the province.

Idlib is held by a coalition of mostly hard line Islamist rebel factions including al-Nusra Front, al-Qaeda's Syrian branch.

Al-Nusra and the Islamic State extremist militia are excluded from a fragile truce that was brokered by the United States and Russia and went into effect in most of Syria in February.