Railway officials in Pakistan say at least twenty people have been killed in a bomb explosion on a crowded passenger train near the town of Khairpur in Sindh province.

They say more than seventy-five people were injured.

The bomb went off in the early hours of the morning while most passengers were asleep.

Rescue teams have taken the injured to a nearby hospital, and medical teams have been sent from Karachi to help treat them.

The BBC correspondent in Islamabad says there's no indication yet who planted the bomb, which he says is by far the most serious in a number of attacks in the past week.