Syrian opposition leaders started leaving Geneva after air strikes on markets in the northwest killed at least 44 people, saying they could not take part in peace talks while civilians were dying daily.

In some of the deadliest violence since a ceasefire took effect in February, a suspected regime bombing raid on Tuesday hit a market in the city of Maaret al-Numan, killing at least 37 civilians, a monitor said.

Footage showed bloodied bodies scattered among twisted metal stalls in a street strewn with fruit and vegetables.

Another strike on a fish market in the nearby town of Kafranbel killed seven civilians, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.

The strikes were in Idlib province, which is under the control of Al-Qaeda's Syrian offshoot Al-Nusra Front.

Like the Islamic State group, Al-Nusra is excluded from the ceasefire and regime forces have continued offensives in areas under its control.

The main opposition High Negotiations Committee condemned the strike on Maaret al-Numan as a "massacre" and clear violation of the truce.

"It is a dangerous escalation of an already fragile situation, showing contempt for the whole international community at a time when there is supposed to be a cessation of hostilities," said spokesman Salem al-Meslet.

The raid was "Assad's response" to the HNC's decision to suspend its formal participation in negotiations.