At least 11 people were killed and 22 others wounded when a five-storey residential complex collapsed in Pakistan's port city of Karachi on Thursday, leaving additional victims trapped in the rubble.

The building had initially been constructed as a four-storey complex, but another floor was added about a year ago, in violation of construction rules, officials said.

"The dead included seven women and three children," Qarar Abbasi, police surgeon at the government-run Abbasi Shaheed Hospital, told AFP.

Rescuers search for victims in the rubble of a collapsed residential building in the Pakistani city of Karachi on Thursday
Rescuers search for victims in the rubble of a collapsed residential building in the Pakistani city of Karachi on Thursday AFP / Asif HASSAN

Two houses adjoining the building also collapsed.

Sama Kausar, a senior health official, confirmed the death toll and told AFP that several people were trapped inside the rubble, with rescuers struggling to reach them.

A building inspector said the sewage system appeared to have triggered the collapse, but a full technical enquiry would be conducted.

Roof and building collapses are common across Pakistan mainly because of poor safety standards and shoddy construction materials in the South Asian country of 200 million.

Pakistan