An overloaded long-haul bus veered off a bridge and plunged into a river outside the northern Nigerian city of Kano on Sunday, killing 56 people, according to road safety officials.

Only six passengers were rescued from the accident and were rushed to a nearby hospital, Hammed Ibrahim of the Federal Road Safety Commission (FRSC) in Kano told reporters.

The accident occurred in the early hours of the morning, possibly because the driver, who was among the dead, dozed off after an overnight journey, said Ibrahim.

Multiple-casualty accidents are common on Nigerian roads. Poor conditions of roads and vehicles plus speeding, overloading and reckless driving are blamed to be the main causes.