LAHORE (AFP) - A blast on board a passenger train in central Pakistan on Monday wounded at least eight people, police said, though the nature of the explosion was not immediately clear.

Police said the explosion happened in the washroom of a carriage on the Shalimar Express from the eastern city of Lahore to Karachi and they were investigating whether a bomb or gas cylinder was to blame.

"At least eight people have been wounded, condition of one of the victims is critical," Shehzad Asif, district police chief of Toba Tek Singh city, where the incident took place, told AFP.

"Bomb disposal experts are investigating the nature of the blast," Mr Asif said.

Mr Rao Asif Zaman, another police official on site, said the blast had damaged the carriage but the track remained safe.