The banned Tehrik-e-Taliban Pakistan (TTP), with which the government is negotiating to end a bloody insurgency, claimed responsibility for the attack outside a police training centre.
The bomber targeted the bus as it was coming out of the centre in Shah Latif Town on the outskirts of Karachi. "The bus was taking policemen for security duties when the incident took place," SSP Aamir Shaukat said.
Seemi Jamali, a top official at Jinnah Hospital, said 11 policemen were killed. Rescue officials said two civilians were among the dead.
TTP spokesman Shahidullah Shahid claimed responsibility for the attack while speaking to Dawn News channel. He said it was a "revenge attack" for the killing of militants.
Also Read
Taliban fighters were victims of targeted killings at several places in Khyber-Pakhtunkhwa province, he claimed. "Our defensive war will continue until an agreement is reached on a ceasefire," he said.
Teams representing the government and the TTP have held discussions on framing a roadmap for a dialogue but it is unclear whether the two sides will agree on a ceasefire.
The TTP and other militant groups have carried out similar bomb attacks on police and paramilitary personnel in Karachi in the past.