Pakistani President Pervez Musharraf has dismissed fears that his country's nuclear weapons could be acquired by Islamist militants.
The BBC's Syed Shoaib Hasan asks if Pakistan's nuclear arsenal is really that secure.
A report last year recommended that the US send in special forces to help "secure the Pakistani nuclear arsenal".
It is not clear how many nuclear weapons Pakistan has
Pakistan's foreign office dismissed the report as "outlandish musings", insisting there was no danger of the country's strategic assets falling into the wrong hands.
At the moment, few believe Islamists could take power in Pakistan.
But there has been huge concern over Pakistan's nuclear facilities since 2004.
That was when the "father of Pakistan's nuclear bomb", AQ Khan, confessed to leaking nuclear secrets to Iran, North Korea and Libya.
He received a presidential pardon and has since been under house arrest. Pakistan's government says he has revealed the full extent of his activities.
'US actively planning'
Nevertheless, one leading nuclear expert in Pakistan, Dr Pervez Hoodbhoy, says the US is right to have worries.
AQ Khan - Pakistan will not let Western officials question him
"The US has been actively planning contingency measures as Pakistan's nuclear weapons are and will remain a major concern."
Estimates of the number of weapons Pakistan has vary from 40 to more than 100 warheads.
Once upon a time, the received wisdom was that Pakistan needed three bombs, to attack Delhi, Mumbai and Calcutta in neighbouring nuclear rival, India.
Dr Hoodbhoy says more weapons means more people having access to the weapons facilities.
But he believes the actual weapons are safe.
"As far as the weapons themselves are concerned, I don't believe they can be obtained by fundamentalist groups like al-Qaeda.
"The days of smuggling centrifuges out of Kahuta [Pakistan's main nuclear research facility] ended with AQ Khan."
Another nuclear expert, Brig Shaukat Qadir, agrees on that point.
"Pakistan's nuclear weapons are only as much at risk as those of the US or India," he says.
"There are differing layers of security and everyone is checked and double checked while entering and leaving the facility."
According to Brig Qadir, even highly trained troops would find it almost impossible to storm Pakistan's nuclear facilities.
President Musharraf pardoned AQ Khan
"In the first place there is the secrecy surrounding the actual weapons storage and development facilities," he says.
"For example, while everybody talks about Kahuta, it is no longer the main facility."
Then, he says, the way the nuclear facilities were built makes penetration nearly impossible.
Facilities like Kahuta are built hundreds of feet underground.
Dr Hoodbhoy agrees that Pakistan has taken steps to increase the safety of its nuclear weapons. These include sending personnel who guard the facilities for training in the US.
Even so, he says the country's nuclear weapons security is "not foolproof".
"Even though the weapons themselves are secure, that is not as true of the fissile material."
He believes small amounts of enriched uranium or plutonium could be smuggled out of Pakistan's nuclear facilities.
"You need about 25kg to make a device the size of [that used at] Hiroshima," he says, adding that making the actual bomb is relatively easy.
"It is getting the weapons grade material which requires industrial facilities."
Brig Qadir disagrees: "Everybody understands the fissile material is the main component... do you really think it will be as readily available as that?
"Both the weapons and the fissile material are accorded the same level of security. The material, therefore has the same chance of being stolen as the weapons."
Bin Laden 'meeting'
Dr Hoodbhoy says there is another worry.
Bin Laden - reported to have met Pakistan nuclear expert in 2001
"A renegade, or set of renegade Pakistan nuclear scientists could help al-Qaeda or another such group develop a device," he says.
"There are people within [the Pakistani establishment] who still believe that force is the only answer to our troubles."
Some observers recall what is referred to as a one-off meeting in local intelligence circles that took place in Afghanistan in the summer of 2001.
Among those said to have been at the Kandahar meeting were several Pakistanis, including a recently retired top nuclear weapons scientist.
Also reportedly present was al-Qaeda leader Osama Bin Laden, who is said to have been intensely interested in how to acquire nuclear weapons.
Bin Laden heard that making the bomb was relatively simple, one witness later told officials.
Obtaining weapons grade uranium ore or plutonium was the problem, he was reportedly told.
At that point came a query that still worries Western security experts.
Bin Laden is reported to have asked: What if we already had the ore?
After the 11 September 2001 attacks on the US, all the Pakistanis said to have attended the meeting were questioned by Pakistani and US security agents.
Some were released, some remain in custody. Details of the investigation were never made public.
"I know some of the scenarios I have talked about seem outlandish," says Dr Hoodbhoy.
"But for the sake of Pakistan, and everyone else, they should be taken seriously - if 9/11 can happen, so can this."