But US officials worry their limited knowledge about the location of the arsenal could pose a problem, it said, a week after Pakistani President Pervez Musharraf declared a state of emergency.

"We can't say with absolute certainty that we know where they all are," one unidentified former US official told the newspaper, adding that any US effort to secure Pakistan's nuclear arsenal "could be very messy."

Under a more optimistic scenario, the Pakistani military would help the United States in any intervention, the Post said. In other cases, that assistance might not be forthcoming, it cautioned.

The report said US officials would not discuss details of the classified plans, "but several former officials said the plans envision efforts to remove a nuclear weapon at imminent risk of falling into terrorists' hands."

US officials and lawmakers have voiced alarm that the Musharraf government could lose control over its nuclear arsenal amid the mounting political crisis there.

"I'm very concerned about it. Not immediately, but over the next year to two years," Senator Joseph Biden, a Democratic presidential contender, said on CNN.

Biden, chairman of the Senate's Foreign Relations Committee, said the United States needed to shore up anti-Musharraf moderates or risk seeing Pakistan go the way of Iran three decades ago.

"The Shah got overthrown and moderates got crushed by extremists," he said.

But Richard Armitage, who as deputy secretary of state led the US effort to get Musharraf on board the anti-terror struggle after the September 11 attacks of 2001, dismissed fears over the safety of Pakistan's nuclear arsenal.

"You know as well as I do that that nuclear arsenal is one, dispersed, and second, carefully guarded by the army," he told CNN.

"Now we have had, historically, discussions with the Pakistani army about the safeguarding of those nuclear weapons," the former official said.

"So I think in the short or even medium term, should things turn badly, we are not going to worry about nuclear weapons in the first instance."

Lieutenant General Carter Ham, director of operations with the US Joint Chiefs of Staff, said Wednesday that the Pentagon was keeping a close eye on Pakistan's nuclear arsenal.

"Any time there is a regime that has nuclear weapons and that experiences a situation like in Pakistan, of course there is a primary concern," he told reporters.

Islamabad, Washington's key ally in the fight against Al-Qaeda and the Taliban, has amassed about 50 nuclear weapons since detonating its first atomic devices in May 1998 in a series of tit-for-tat tests with India.

Pakistan also is suspected of selling atomic secrets on a global black market headed by its disgraced chief nuclear scientist Abdul Qadeer Khan.

The Post recalled US allegations that two retired Pakistani nuclear scientists traveled to Afghanistan in August 2001 to brief Al-Qaeda leader Osama bin Laden about how to make nuclear weapons.

Among US intelligence agencies, there is particular concern now over the cohesion of Pakistan's army if extremist violence and opposition protests against Musharraf escalate, the report said.

"If there is a collapse in the command-and-control structure -- or if the armed forces fragment -- that's a nightmare scenario," said John Brennan, a retired CIA official and ex-director of the National Counterterrorism Center.

"If there are different power centers within the army, they will each see the strategic (nuclear) arsenal as a real prize," he told the Post.

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