Pakistani officials say an explosion on a bus in the southern Pakistani city of Karachi has killed at least four people and wounded more than 40.
Seemi Jamali, a doctor at Jinnah Hospital in Karachi, said eight of the wounded were in critical condition.
Police say they are investigating whether the cause of the blast was a bomb or the explosion of a compressed-natural-gas cylinder.
Pakistan's commercial capital and biggest city has seen numerous militant attacks over the past 10 years and is also plagued by violence between rival ethnic-based factions.
With reporting by Reuters