It was not immediately clear what caused the blast, but it appeared to be accidental, probably caused by the ignition of a cooking gas canister.
Troops stand guard while rescue workers look for survivors amid the rubble of a damaged building following the explosion in Karachi (Fareed Khan/AP)
Some people were still trapped under the rubble, according to Saad Edhi, a spokesman for the Edhi ambulance service.
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Sajid Sadozai, a senior police officer in Karachi, confirmed that three people had died.
TV footage showed rescuers digging through the remains of a collapsed building.
