Concerned about the safety of Pakistan's nuclear weapons in the wake of the growing unrest in the Islamic nation, the United States has prepared a contingency plan to safeguard them, but senior officials are worried over their limited knowledge about the location of the arsenal.

'We cannot say with absolute certainty that we know where they all are. If an attempt were made by the United States to seize the weapons to prevent their loss it could be very messy,' The Washington Post reported on Sunday, quoting a senior official as saying.

Of the world's nine declared and undeclared nuclear arsenals, none provokes as much worry in Washington as that of Pakistan's as some of that country's territory is partly controlled by insurgents bent on committing hostile acts of terrorism in the West.

Moreover, as the Musharraf government in Islamabad is the least stable, the Bush administration is worried that Pakistan's nuclear arsenal might be used in a 'horrific war' with India than it becoming a security threat to the US in the event of any theft or diversion to terrorist groups.

'Because the risks are so grave, the US intelligence officials have long had contingency plans for intervening to obstruct such a theft in Pakistan,' another senior official was quoted by the daily as saying.

The plans imagine, in the best case, that Pakistani military officials will help the Americans eliminate that threat. 'But in other scenarios there may be no such help,' said Matt Bunn, a nuclear weapons expert and former White House official in the Clinton administration.

'We are a long way from any scenario of that kind. But the current turmoil highlights the need for doing whatever we can right now to improve cooperation and think hard about what might happen down the road,' the daily quoted him as saying.

However, former and current administration officials said that they believed that Pakistan's stockpile 'is safe.'

But they worry that its security could be weakened if the current turmoil persists or worsens. They are particularly concerned by early signs of fragmented loyalties among Pakistan's military and intelligence leaders, who share responsibility for protecting the arsenal.

'The military will be stretched thin if the level of protest rises,' said John E McLaughlin, a top official at the CIA from 2000 to 2004.

'If the situation becomes more volatile, the conventional wisdom (about nuclear security) could come into question.'

He noted that Pakistan's army had become increasingly diverse, reflecting the country's ethnic and religious differences, 'and that was different from the way it was years ago.'

It may be mentioned that when the United States learnt in 2001 that Pakistani scientists had shared nuclear secrets with members of Al Qaeda [Images], the Bush administration had responded with tens of millions of dollars worth of equipment such as intrusion detectors and ID systems to safeguard Pakistan's nuclear weapons.