Rescue workers pulled out the survivors and bodies from under the rubble in Liaquatabad town and shifted them to a hospital, several other residents are feared stuck in the rubble.
Four bodies, including those of a woman and a child, were recovered from the debris. An eyewitness said that 20 people were residing in the building, the Geo News reported.
According to area residents, the 60-80 yard building was in a derelict state and the third storey had been illegally built a few months earlier.
The collapsed building was not part of the list of Karachi's 300 dilapidated buildings, Mustafa said.
People continue to live in dangerous buildings across the city, despite notices being issued by the authorities concerned.
Safety codes are often ignored in Pakistan and in some of the poorer areas construction is haphazard and bribes are paid to inspectors to approve substandard construction.