An audacious weekend assault by Islamic militants on Pakistan's army headquarters is again raising fears of an insurgent attack on the country's nuclear weapons installation.
Pakistan has sought to protect its nuclear weapons from attack by the Taliban or other militants by storing the warheads, detonators and missiles separately in facilities patrolled by elite troops.
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 programme at London think tank Chatham House. "There are enough command-and-control mechanisms in place to prevent that."
A US 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 authorised 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 Centre.
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.