An explosion on a bus in the Pakistani city of Karachi has killed six people and wounded 48 others.

Pakistan's commercial capital and biggest city has seen numerous militant attacks over the past ten years and is also plagued by violence between rival ethnic-based factions.

The bus sustained serious damage in the explosion and subsequent fire.

While police said the bomb had been planted on the bus, provincial official Sharfud Din Memon said it was left on a motorbike and went off as the bus passed.

Eight of the wounded were in critical condition, according to a doctor at Jinnah Hospital.