Analysts are divided on how secure these weapons are. Some say the weapons are less secure than they were five years ago, and Saturday's attack would show a "worrisome" overconfidence by the Pakistanis.
While complex security is in place, much depends on the Pakistani army and how vulnerable it is to infiltration by extremists, said a Western government official with access to intelligence on Pakistan and its nuclear arsenal, speaking on condition of anonymity because of the sensitivity of the subject.
Analysts say a more realistic scenario would involve militant sympathizers getting work as scientists at the facilities and passing information to extremists.
"It's not thought likely that the Taliban are suddenly going to storm in and gain control of the nuclear facilities," said Gareth Price, head of the Asia program at London think tank Chatham House. "There are enough command-and-control mechanisms in place to prevent that."
A U.S. counterproliferation official in Washington said strong safeguards are in place and there is no reason to believe the nuclear arsenal is in imminent jeopardy of seizure by militants.
The official, who commented on condition of anonymity because he was not authorized to speak on the matter publicly, said there is a major difference between attacking a nuclear site and actually seizing and using the nuclear material stored inside.
Security at Pakistan's isolated nuclear installations is believed to be significantly higher than at the army headquarters, which was relatively relaxed by the standards of other nations. Thousands of people and vehicles enter the headquarters compound in Rawalpindi daily, and the 10 attackers, while able to take dozens of hostages Saturday and kill 14 people before a commando raid ended the siege, never penetrated to the heart of the complex.
Pakistan is estimated to have between 70 and 90 warheads, according to Hans Kristensen, director of the Nuclear Information Project of the Federation of American Scientists.
Shaun Gregory, an expert on Pakistani security at the University of Bradford in Britain, said militants have struck near an air base in Sargodha, where nuclear missiles are believed to be stored, and the Wah cantonment, where missiles that could carry nuclear weapons are believed to be assembled. He added that the attacks did not appear to have targeted nuclear weapons.
Pakistan uses armed forces personnel to guard nuclear weapons facilities, and it physically separates warhead cores from their detonation components, Gregory wrote in the July issue of The Sentinel, the monthly journal of the Combating Terrorism Center.
The components are stored in protected underground sites. The warheads themselves are electronically locked to ensure that they cannot be detonated even if they fall in terrorists' hands, Gregory said.
The Pakistan military carefully screens and monitors the officers vested with protecting the warheads, drawing them almost exclusively from Punjabi officers who are considered to have fewer links to religious extremists or with the Pashtun area of Pakistan, where the Taliban garners much of its support.
No action or decision involving a nuclear weapon can be undertaken by fewer than two persons. But Gregory acknowledged the possibility of collusion between cleared officers and extremists.
The personnel assigned to sensitive nuclear posts go through regular background checks conducted by Pakistan's intelligence services, according to a 2007 article in the journal Arms Control, co-written y Naeem Salik, a former top official at Pakistan's National Command Authority, which oversees the nuclear arsenal.
"It is being acknowledged by the world powers that the system has no loopholes," Maj. Gen. Athar Abbas, a military spokesman, said Monday. "The system is foolproof, as good and bad as their own systems."
The U.S. and the British governments agree there is little risk of a weapon falling into militants' hands.
In London, British Foreign Secretary David Miliband said there is no evidence "that has been shown publicly or privately of any threat to the Pakistani nuclear facilities, said.
Gregory said in an e-mail to The Associated Press that he did not share Miliband's assertion, adding that "there is plenty of evidence of threat."
Individuals in the Pakistan military have colluded with al-Qaida in providing safe houses for Khalid Sheikh Mohammed, the alleged mastermind of the 9/11 terrorist attacks, and individuals in Pakistan's civil nuclear sector have met with al-Qaida figures, including Osama bin Laden himself, Gregory said.
U.S. Secretary of State Hillary Rodham Clinton dismissed any suggestion militants could overthrow the government and gain control of the nuclear arsenal. "We have confidence in the Pakistani government and military's control over nuclear weapons," she said.
Kristensen said that while U.S. officials have said they have helped Pakistan increase security at its nuclear facilities, "they have not been allowed to go to those sites, so it's something they've had to do remotely."
Saturday's attack "somehow seems to show that the Pakistani military is perhaps a little overly confident" about some of its most important military facilities, he said.
"If a relatively small group of people is able to penetrate into their 'Pentagon,' then it might show something about the overconfidence of the Pakistanis, and that is worrisome _ it's surprising that they were able to go in there relatively simply," Kristensen said.
He noted that the military headquarters is different from a nuclear facility. "One cannot compare insurgents going into an office building to them going into a nuclear facility for the nation's crown jewels," he added.
While stringent security checks on personnel are meant to prevent militant sympathizers from working at the facilities, Pakistan's nuclear establishment has seen serious leaks of nuclear knowledge and materials by insiders.
Top government scientist A.Q. Khan operated a global black market nuclear network for more than a decade until he was uncloaked by U.S. intelligence. And the CIA has confirmed a meeting between Khan associates and bin Laden before 9/11.
Israel has not taken a formal position on the danger of Pakistan's nuclear weapons falling into the hands of terrorists. However, in a parliamentary briefing last year, Defense Minister Ehud Barak mentioned such a scenario as a nightmare for the world, according to security officials speaking on condition of anonymity because the session was closed.
"Pakistan's weapons are less secure today than they were five years ago, and it seems they're even less secure than under the Musharraf government," said Gerald Steinberg, professor of political studies and conflict management at Bar Ilan University in Israel, referring to the previous administration of President Gen. Pervez Musharraf.
Steinberg said Israelis are becoming less confident of the U.S. ability to control events and put plans into action that would protect Pakistan's nuclear stockpile.
Hess reported from Washington. Associated Press writers Jill Lawless in London, Ravi Nessman in Islamabad and George Jahn in Vienna contributed to this report.