Overnight air strikes killed 19 civilians, seven of them children, in a residential district of a town in northwestern Syria, a monitor said on Wednesday.

"Suspected Russian air strikes targeted several houses in Maarshurin," a rebel-held town in Idlib province, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

He said 13 members of a single family were among the dead.

Residents of Maarshurin were still working on Wednesday morning to clear rubble from buildings hit in the bombardment, an AFP photographer reported.

One had been completely flattened and blood stained the rubble.

"These are all civilian homes," said resident Zahed Abu Abdo.

The Observatory said more than 25 people were wounded in the strikes, some of whom were in critical condition.

The Britain-based war monitor relies on a network of sources inside Syria and says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.

Russia, which has been carrying out air strikes in support of President Bashar al-Assad's government since 2015, denied responsiblity on Wednesday.

"Russian warplanes did not carry out sorties in this zone," its defence ministry said, according to the Interfax news agency.

Idlib province is one of four regions of Syria which are supposed to be safe areas under a deal agreed by Russia, fellow regime ally Iran, and rebel backer Turkey in May.

Maarshurin is jointly controlled by Hayat Tahrir al-Sham, a rebel alliance dominated by Al-Qaeda's former Syria affiliate, which is not covered by the deal.

More than 340,000 people have been killed since Syria's conflict erupted with anti-government protests in 2011.

More than half the population have been forced out of their homes, including tens of thousands who have sought refuge in Idlib province.