Nov 8 (Reuters) - Pakistan's nuclear installations are believed to be well enough guarded to prevent a successful attack by Islamist militants, though this remains a "low probability, high consequence" risk.
Here are some facts about Pakistan's nuclear programme:
Pakistan began a programme to obtain nuclear weapons after its defeat by India in a 1971 war which led to the division of the country and the creation of Bangladesh, then East Pakistan.
The army sees its nuclear weapons as essential to offset the conventional superiority of its much bigger neighbour.
India sees its own nuclear weapons as a deterrent against Pakistan and China, which defeated it in a border war in 1962.
It tested nuclear weapons in May 1998, shortly after India announced it had done so.
Both countries faced international sanctions as a result, although India has since won effective recognition as a nuclear power following an accord negotiated with the United States.
Neither Pakistan nor India have signed the nuclear Non-Proliferation Treaty (NPT).
Estimates vary on the size of Pakistan and India's nuclear arsenals, although analysts suggest India has 70-120 nuclear weapons while Pakistan has 60-120 weapons.
These can be delivered by aircraft, or by missiles, which both countries have been developing and testing. Analysts believe the nuclear weapons have reduced the likelihood of a conventional war between India and Pakistan.
At the same time, they have opened the way to unconventional proxy wars, as happened between the United States and Soviet Union during the Cold War. India accuses Pakistan of using its nuclear umbrella as a cover for what it calls cross-border terrorism by Islamist militants, a charge Islamabad rejects.
Pakistan, which has fought three full-scale wars with India, including two over the former kingdom of Jammu and Kashmir, is believed to have prepared its warheads for deployment twice.
The first time was in 1999 during the Kargil conflict, fought in the mountains on the Line of Control, the ceasefire line dividing Jammu and Kashmir.
The second was during a military standoff between India and Pakistan in 2001/2002 which followed an attack on the Indian parliament in December 2001, blamed on Pakistan-based militants. India has a policy of no first use of nuclear weapons but has made clear that if it were hit by a nuclear bomb from Pakistan it would strike back in force.
Pakistan has indicated it would use nuclear weapons if it believed its existence were threatened in a conventional war. Recent growth in its nuclear programme has been seen as an attempt to develop a second strike capability against India.
Pakistan is believed to have worked closely with the United States to build elaborate security mechanisms to prevent Islamist militants from seizing nuclear material.
The nuclear programme is controlled by the army and run by the Strategic Plans Division headed by General Khalid Kidwai.
Pakistan has copied "best practice" on security from the United States, including on personnel vetting.
Analysts say it is believed to have developed its own Permissive Action Link (PAL) system, modelled 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, deception and secrecy.
Reassurances from the U.S. administration that it is confident about the security of Pakistan's nuclear weapons have underpinned views that Washington has worked closely with Pakistani authorities on this.
However, analysts also say Pakistan is likely to have kept at least some of its nuclear programme secret given its concerns that an outside power might try to neutralise its nuclear weapons in the event of war. (Reporting by Myra MacDonald; Editing by Matthew Jones) (For analysis, double-click on [ID:nL4229969]) For additional sources see: Nuclear Threat Initiative website: here here