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

The Syrian Civil Defense, known as White Helmets, says 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 3 million people. The U.N. has warned its humanitarian operations in the region are at risk.