Concerned that Usama bin Laden is seeking to get his hands on nuclear weapons, the United States has dropped its punitive measures against Pakistan's nuclear program and is now offering to advise the country on securing its stockpile.

The Americans spent a decade sanctioning Pakistan for building nuclear weapons, but that policy effectively changed with the Sept. 11 terror attacks on the United States.

The United States now views Pakistan as an essential ally in the war against terrorism. The Americans want to cooperate with Pakistan on nuclear issues to ensure that no nuclear material leaks to bin Laden's Al Qaeda network or comes under the control of Islamic fundamentalists inside Pakistan.

President Bush lifted economic sanctions originally imposed in 1990 by his father. And when Secretary of State Colin Powell arrived last month, he went a step further, proposing that the United States provide training for Pakistan's nuclear facilities.

"During his visit, Colin Powell offered us that kind of support, to train Pakistanis in America on the safeguarding of nuclear installations," said Pakistani Foreign Minister Abdul Sattar.

Asked if Pakistan had accepted, Sattar responded, "who would refuse?"

Neither Pakistan nor the United States has released details. But the offer is believed to include training on everything from preventing accidents at civilian power plants to guarding against the theft of weapons-grade uranium, said Rifaat Hussain, head of the department of defense and strategic studies at Quaid-i-Azam University in Islamabad.

Powell, speaking Wednesday in Washington, said Pakistani President Pervez Musharraf "understands the importance of ensuring that all elements of his nuclear program are safe and secure."

Musharraf "knows that if he needs any technical assistance in how to improve that security level, we would be more than willing to help in any way that we can," Powell added.

The shift in U.S. policy does not mean American concerns about Pakistan's nuclear program have eased. If anything, the United States may more worried than ever about an arsenal that includes an estimated 20 to 30 warheads. Pakistan has never said how many weapons it has.

The Americans have three big concerns about Pakistani nuclear weapons: the spread of nuclear material to terrorist groups, the prospect of Islamic fundamentalists taking power in Pakistan, and the fear of a nuclear war between Pakistan and archrival India.

How serious is each threat?

— Secretary of Defense Donald Rumsfeld said Thursday that bin Laden's network has been trying for years to acquire weapons of mass destruction.

Rumsfeld named no countries. However, speculation has focused on Pakistan, which until the Sept. 11 attack had backed Afghanistan's ruling Taliban movement, which in turn has harbored bin Laden.

There's also a widespread belief that the former Soviet Union, with its widely scattered nuclear program, impoverished scientists and soldiers and often lax security, would be the best place to look for a stolen nuke.

Yossef Bodansky, a former consultant to the U.S. State and Defense Departments and author of Bin Laden: The Man Who Declared War on America, wrote that bin Laden has tried but failed to acquire weapons of mass destruction in several parts of the former Soviet Union, including Russia, Kazakstan, Ukraine and Chechnya.

Politics are turbulent in Pakistan, but the country has kept a tight lid on nuclear materials and technology since it launched the program in the mid-1970s, noted Hussain, the analyst.

He said Pakistan is proud of being the only Islamic country to build nuclear bombs, and has rebuffed efforts by other Islamic countries, including Iran and Libya, to acquire technology and material.

Last week, Pakistan arrested two retired nuclear scientists, Sultan Bashir-ud-Din Mehmood and Abdul Majid. But the government insisted they were being questioned about alleged pro-Taliban sympathies, not about passing on nuclear secrets. Both were released, but a presidential spokesman said they were called in again Saturday for questioning.

— Pakistan's history of military coups has raised fears that Islamic fundamentalists in the officer corps could someday seize power, thereby gaining control over Pakistan's nukes.

Musharraf, who came to power in his own coup two years ago, recently purged the senior military ranks of officers viewed as Islamic fundamentalists. Five of the top 14 officers were moved to lesser positions.

"This threat has receded," said Hussain. "Anyone harboring these kinds of ideas has been sidelined."

Islamic parties have been staging noisy street protests against Musharraf's decision to abandon the Taliban and side with the United States.

However, the parties have never fared well in elections, and throughout Pakistan's 54-year history, its leaders have sought close ties with the United States and the West.

— Nuclear tension between Pakistan and India has created several crises in the past decade and many believe it remains the greatest threat to the region.

The countries conducted back-to-back nuclear tests in 1998, and a year later were fighting yet again over the disputed Himalayan region of Kashmir.

Both countries have nuclear weapons that could be delivered by warplanes or missiles. However, neither has the "push-button" capability to launch, according to Aslam Beg, a Pakistani retired army chief.

Pakistan keeps its nuclear warheads separate from the other components of the weapon, Beg said, adding that the bomb would first have to be assembled, and then launched from either from a missile or a plane.

"There would be a gap of hours, or even days before it could be put together," said Beg.

Pakistan and India remain archenemies, exchanging artillery fire almost daily across the disputed frontier in Kashmir. However, they have agreed not to target each other's nuclear facilities, and even hard-liners such as Beg believe the existing tensions aren't an insurmountable obstacle to progress on the nuclear issue.

Indian Defense Minister George Fernandes, normally a harsh critic of Pakistan, even had a kind word to say this week about Pakistan's handling of its nuclear program.

"Politics apart, I must give [the Pakistanis] credit. They are responsible people and will not allow people to walk away with nuclear weapons," said Fernandes.