Pakistani volunteers, security personnel and residents search for the victims in the rubble of a collapsed a five-storey residential building in Karachi. AFP

Soldiers have been deployed to help in the search efforts after a five-storey building and two adjoining houses collapsed in the port city of Karachi on Thursday.

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

A building inspector said the sewage system appeared to have triggered the disaster, but a full technical investigation would be carried out.

"The death toll in the building collapse rose (from 11 on Thursday) to 16, after five more bodies were pulled out of the rubble today," Salma Kauser, a senior health official in Karachi, told reporters.

An official at the Abbasi Shaheed hospital confirmed the toll.

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Rehan Hashmi, a senior municipal official, said the army had been deployed to help with the rescue and recovery mission.

"Seven people are still missing, but it is premature to say whether they were trapped under the debris or not," Hashmi said.

Narrow alleyways have restricted the access of heavy machinery to the disaster site, meaning much of the rubble is being cleared manually.

Authorities have sealed off the area, with soldiers standing guard and blocking curious onlookers in the city's central district.

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.