KARACHI, Pakistan—A powerful gas explosion in a sewage system in a southern Pakistan city killed at least 10 people and injured 13 others Saturday, police and a health official said. Police spokesman Sohail Jokhio said the blast was apparently caused when something ignited gas that had accumulated in the sewer beneath a bank building in the Shershah neighborhood of the port city of Karachi. Jokhio said it was not yet clear what ignited the gas but a team of explosives experts had been summoned to investigate. Dr. Sabir Memon at Trauma Center Karachi said 10 people were killed and 13 others were injured, at least three critically. He said several injured were sent to the intensive care unit. Jokhio said windows were shattered in nearby buildings and a parked vehicle nearby was also badly damaged. Many sewage channels in the city have been covered, mostly illegally, by constructing concrete structures …