A series of airstrikes hit a residential district of a town in northwestern Syria overnight, killing 19 civilians, seven of them children, a monitor said on Wednesday.

"Suspected Russian airstrikes targeted several houses in Maarshurin," an opposition-held town in Idlib province, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

Syrian activists in the area said the Assad regime has been carrying out intensified strikes on opposition-held areas over the past few weeks.

Idlib has an estimated 2 million residents, including those displaced from other provinces.

It is the only province in Syria under opposition control. Syria's crisis began with peaceful anti-regime demonstrations in March 2011.

The conflict soon spiraled into a multi-sided civil war that has claimed hundreds of thousands of lives and displaced about half of Syria's pre-war population of 22 million.