Syrian government forces have launched heavy air strikes on rebel-held areas in Hama province, to counter a major rebel assault there.

The monitoring group, Syrian Observatory for Human Rights, said the assault that began on Tuesday is the biggest coordinated rebel offensive in the province since 2014. It also said at least 25 people including six children had been killed in government air strikes overnight.

Syrian media said the air force had carried out "concentrated strikes" against what it described as terrorists in the area.