FACTBOX: Pakistan's nuclear arsenal

(Reuters) - Some security experts fear Pakistan's nuclear weapons could be compromised or fall into the hands of Islamic militants as the country's instability deepens in the wake of Benazir Bhutto's assassination.

Here some details about Pakistan's nuclear capability:

* NUCLEAR SAFETY - WHAT COULD HAPPEN?

-- Pentagon officials say Pakistan's nuclear arsenal is secure in military hands, but some U.S. lawmakers and experts warn that nuclear material and designs could leak out if political instability persists.

-- Musharraf has concentrated control over Pakistan's nuclear weapons but a decline in his support within the military amid the current political crisis could raise a risk that control over the weapons could weaken.

-- That could open the door to theft or sale of weapons material to extremist groups, some experts say.

-- The military is suspected by some of at least knowing about the smuggling activities of Pakistan's A.Q. Khan network, which sold weapons technology to Iran, North Korea and Libya.

* WHAT IS KNOWN ABOUT PAKISTAN'S ARSENAL?

-- Pakistan embarked on a nuclear weapon program in the early 1970s after its defeat and break up in the 1971 Indo-Bangladesh war. Islamabad regards nuclear weapons as essential to safeguard the South Asian balance of power and offset its conventional inferiority and lack of strategic depth against India.

-- Pakistan conducted nuclear tests in May 1998, shortly after India conducted its own weapon tests and declared itself a nuclear weapon state. Pakistan is not a signatory to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT).

-- Pakistan is believed to have stockpiled approximately 580-800 kg of highly enriched uranium (HEU), sufficient to build 30-50 fission bombs. In 1998, Pakistan commissioned the Khushab research reactor, which is capable of yielding 10-15 kg of weapons-grade plutonium annually.

* GREATER CAPABILITY:

-- The Washington based Institute for Science and International Security reported last June that Pakistan appeared to be building a third plutonium production reactor at its Khushab nuclear site.

-- They said the activity at Khushab and also at a plutonium separation facility at Chasma, 50 miles west of Khushab, "should be viewed as a sign of an accelerated nuclear arms race" between Pakistan and its nuclear-armed adversary, India.

Sources: Reuters/www.nti.org (Nuclear Threat Initiative)

(Writing by David Cutler, London Editorial Reference Unit; editing by Matthew Tostevin)