An assault on a naval base in Karachi is the latest militant attack on military installations in Pakistan, raising fears about the safety of the country's nuclear arsenal.
Pakistan has the fastest growing nuclear arsenal in the world and in a decade could pass France as the fourth-largest nuclear power, so such brazen attacks on secure military establishments -- militants also attacked the General Headquarters in Rawalpindi in 2009 -- give Western leaders nightmares about militants acquiring nuclear materials, or worse, an entire weapon.
Pakistan first tested nuclear weapons in 1998 in response to tests by old rival and neighbour India.
What is the US position?
President Barack Obama said in 2009 he was confident about the security of Pakistan's nuclear arsenal although he was "gravely concerned" about the overall situation in Pakistan because of its weak government.
Despite that, there is a growing concern among US officials that militants might try to snatch a nuclear weapon in transit or insert sympathisers into laboratories or fuel-production facilities.
Pakistani analysts see the mixed signals from the United States as adding to pressure on the government, which the United States wants to see getting to grips with the militant threat.
The killing of Osama bin Laden on May 2 just a couple of hours drive from the capital in the military town of Abbottabad raises fears that he had help from friends in Pakistan's military or spy agencies -- suggesting that al Qaeda sympathisers might also be among those guarding Pakistan's nukes.
What is Pakistan's position?
Pakistan rejects such fears over its nuclear weapons as "misplaced and unfounded" saying it has very robust, multilayered command and control systems.
Many Pakistanis believe the ultimate US aim is to confiscate Pakistan's nuclear weapons and analysts say reports of US fears about nuclear security fuel such conspiracy theories.
Who controls the weapons?
Pakistan does not release details of its nuclear arsenal. Estimates vary on the size of Pakistan's nuclear arsenal, although analysts suggest Pakistan has between 60 and 120.
The weapons are under control of the military's Strategic Plans Division (SPD). During a period of political instability last year the division boosted security at nuclear facilities and launched a public relations offensive to counter what Pakistan regards as scaremongering over nuclear weapon security.
Pakistan maintains there is no chance of Islamist militants getting their hands on atomic weapons.
The SPD is overseen by the National Command Authority headed by the president and with the prime minister as its vice chairman. Main cabinet ministers and the heads of the army, navy and air force are also members of the NCA, which controls all aspects of the nuclear programme, including deployment and, if ever necessary, their use.
Where are the weapons?
The weapons, designed to be delivered by missiles or fighter-bombers, are stored at secure, secret locations, mostly in Punjab province, analysts say, well away from Taliban heartlands in the northwest, although there have been increasing instances of militant attacks and infiltration into the province.
Other nuclear facilities, including the main Kahuta nuclear weapons laboratory, are near the relatively secure capital, Islamabad.
Who guards them?
Pakistan has 10,000 soldiers guarding its facilities and the SPD has its own independent intelligence section. Staff working in nuclear facilities go through an exhaustive vetting process, involving political, moral and financial checks and psychological testing for 10,000 staff working in nuclear facilities, and security monitors keep close tabs on 2,000 scientists working in ultra-sensitive areas.
Although Pakistan's top nuclear scientist was involved in major nuclear proliferation before his network was uncovered in 2004, no conspiracy or plot related to Pakistan's nuclear facilities had ever been uncovered, the SPD has said.
Pakistan's controls are such that orders to abort a mission involving a nuclear weapon could be given at the last second. Even if a rogue pilot were to fire a missile he would not have the code to arm the warhead, according to the SPD.
Analysts say Pakistan is believed to have developed its own Permissive Action Link system, modeled on one used in the United States, to electronically lock nuclear weapons.
It also relies on a range of other measures including physical security, separation of warheads from missiles and warheads from explosive devices.