Syrian opposition activists say government airstrikes and intense shelling on rebel-held areas in northwest Syria have claimed the lives of dozens of people.

The Britain-based Syrian Observatory for Human Rights and the opposition's Syrian Civil Defense said at least 10 civilians have been killed in Saturday's airstrikes and shelling of rebel-held villages and towns in Idlib province.

Syrian state media said militants shelled government-held areas causing material damage.

The Observatory said that Saturday's fighting alone left 26 troops and pro-government gunmen dead as well as eight militants.

Six weeks of violence has driven nearly 300,000 people from their homes. Many are living under olive trees, in tents or unfinished buildings, crammed into overcrowded shared rooms.

More than 300 civilians have been killed since fighting broke out on April 30.