At least 15 people were killed and 16 others were injured in a blast near Paracha Chowk in Karachi's Shershah area on Saturday, officials said.
SHO Zafar Ali Shah said the explosion took place in a nullah (drain) located underneath a private bank, which had been served a notice to vacate the premises so the nullah could be cleaned up.
The bank's building, as well as a nearby petrol pump, was damaged in the explosion, Shah said.
Meanwhile, Dr Sabir Memon, head of Shaheed Benazir Bhutto Trauma Centre at Dr Ruth Pfau Civil Hospital Karachi (CHK), confirmed that 15 people had died in the incident and 16 others injured in the blast were brought to the hospital. He said eight of the injured were discharged after first medical aid.
The doctor attributed the majority of the casualties to injuries caused by fallen debris and said none of the bodies smelled of any explosives or chemicals.
According to Dr Qarar Ahmed Abbasi, additional police surgeon at CHK, 13 of the blast victims were brought dead to the hospital and two others died during treatment.
Earlier, the Karachi police spokesperson said in a statement that a bomb disposal squad was called to examine the site of the explosion, adding that the cause can be determined with certainty once the squad submits its report.
The report, released later in the day, concluded that the blast occurred due to gas leakage and the accumulation of gases in the nullah underneath the building.
The police spokesperson also said no evidence of terrorism had been found at the site of the blast yet.
Earlier, officials reached the site of the explosion and cordoned off the area, Sindh Rangers said in a statement.
Footage of the blast showed a damaged building and debris lying on the ground. Damaged vehicles could also be seen at the site of the explosion.
People could be seen trying to remove the rubble. There were reports of people being trapped under the debris.
Meanwhile, Special Assistant to the Prime Minister on Political Communication Shahbaz Gill said Pakistan Tehreek-i-Insaf MNA Alamgir Khan's father, Dilawar Khan, had also died in the blast.
"We share the grief of Alamgir Khan and other bereaved families," he tweeted, praying for the deceased.
Later in the day, in a message on Twitter, Prime Minister Imran Khan condoled with the families of the victims. He said he was "especially saddened to hear of the loss of our MNA Alamgir Khan’s father who also perished in the blast".
'Construction of buildings on nullahs illegal'
Later, Sindh Information Minister Saeed Ghani said while speaking to the media that according to the details he had received so far, the bank's building was constructed and given on rent by the SITE Association of Industry.
"I am unable to understand that how can an institution construct a building on a nullah and rent it out," the minister said.
Replying to a question, Ghani added that "apparently, and I am not saying this with finality, the SITE Association is responsible for the incident as it constructed the affected building [on a drain] and gave it on rent."
The minister said principally, the construction of any structure on a nullah was not legal.
Saying that all structures constructed on drains in the city should be removed, he added that however, those affected and displaced as a result of the move must also be given compensation.
"It's not as much their fault as of those who allotted the land to them in the first place," Ghani explained. He said that institutions, departments, authorities and officials who allotted land to people on nullahs must be identified so that action could be taken against them.
Later, the Sindh home department also said while speaking to the media that all structures built on nullahs were illegal.
He said he had issued directives for a transparent investigation of the incident, adding that soon a "grand operation" would be launched against encroachments.