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.

This photo posted on Tuesday, May 28, 2019 and provided by the Syrian Civil Defense White Helmets, which has been authenticated based on its contents and other AP reporting. (Photo: AP)

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.