Syrian rescue workers and activists say government warplanes have again bombed the last rebel stronghold in the country, levelling a building and killing five people inside it, including three members of a single family.

The Syrian Civil Defence, known as White Helmets, said its volunteers pulled bodies and survivors from under the collapsed building in Maaret al-Numan, a town in southern Idlib that was hit in an airstrike on Thursday.

The group says a mother and her two children were among those killed while the woman's third son survived.

Activist-operated Baladi News agency also reported that five people were killed.

Syria's government escalated its offensive on the rebel stronghold in northwestern Syria, home to three million people.

The UN has warned its humanitarian operations in the region are at risk.