The bomb turned the carriage into a mangled wreck, the BBC's Owen Bennett-Jones reports.

The Pakistani authorities said the time bomb went off in the early hours of Sunday morning as the Karachi to Peshawar train approached Khairpur in the southern province of Sindh.

It exploded in a packed carriage while most of the passengers were asleep, according to an official.

The blast was said to be so powerful it brought the train to a grinding halt.

Blast brought train to a halt

Rescue teams took the injured to hospitals in the area and in other nearby cities. The local hospital has declared a medical emergency and medical teams have been sent from Karachi to help treat the injured but there are fears many more will die.

The Pakistani Information Minister, Mushahid Hussain, linked the attack with other explosions in Pakistan, including one in Lahore on Friday.

That bomb, which went off at a cinema, killed three people, while at least one person was killed following an explosion at a vegetable market in Hyderabad on Saturday.

In March, a bomb exploded on a train crowded with labourers near Lahore, killing at least nine people. Mr Hussain said an Indian national had admitted planting the cinema bomb and another Indian national had admitted responsibility for bombings earlier in the year.

The BBC Correspondent in Islamabad said the latest incident was by far the most serious.

Although no group has admitted responsibility for the bomb, the Pakistani authorities said they have evidence that India was behind it.