The bomb turned the carriage into a mangled wreck. Owen Bennett-Jones reports.

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

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

The blast was so powerful it brought the train to a halt

Although no group has admitted responsibility for the bomb, the Pakistani authorities told the BBC they had "unimpeachable" evidence it was the work of the Research and Analysis Wing (RAW) of the Indian intelligence service.

However, an Indian foreign ministry spokesman said the allegations were "preposterous, fictitious and baseless".

'Terrorism act'

Mushahid Hussain: "We have no doubt."

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

He accused India of trying to provoke political violence in Pakistan following the recent nuclear tests on both sides of the border and said his government would make the latest evidence available.

"It is a major act of terrorism. We have no doubt, based on a report that we have now, that this pattern is being fomented from across the border by India through its intelligence organisation, RAW.

Mushahid Hussain tells BBC World: "India is trying to divert attention from internal dissension."

"Perhaps the Indians, having failed to prohibit Pakistan earlier after their tests, they were caught by surprise by the Pakistani tests. Now they want to put pressure on Pakistan through these acts of terrorism.

"We have unimpeachable evidence to back up what we are saying."

Powerful blast

The BBC Islamabad Correspondent, Owen Bennett-Jones, said it was not easy to assess these claims. Some recent attacks had taken place in areas hit by factional violence. In the past, India has denied being involved in bombings in Pakistan.

Initially, the Pakistan authorities reportedly said 23 people had been killed and 75 injured. Pakistan railway officials said the latest blast was so powerful it brought the train to a grinding halt.

Previous explosions

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.

A bomb at a cinema in Lahore on Friday 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.

Our correspondent said the latest incident was by far the most serious.

Nuclear fallout

Owen Bennett-Jones: "The bombing does not necessarily affect broader strategic questions like nuclear talks."

The Pakistani claims follow increased tension between Pakistan and India over recent nuclear tests and the disputed province of Kashmir.

The Pakistani Prime Minister, Nawaz Sharif, invited his Indian counterpart, Atal Behari Vajpayee, on Saturday to hold talks to resolve the nuclear test situation.

Mr Sharif said Pakistan has long favoured a meaningful dialogue and that that was still the case despite the series of nuclear tests conducted last month by both sides.