US special forces snatch squads are on standby to seize or disable Pakistan's nuclear arsenal in the event of a collapse of government authority or the outbreak of civil war following the assassination of Benazir Bhutto.

The troops, augmented by volunteer scientists from America's Nuclear Emergency Search Team organisation, are under orders to take control of an estimated 60 warheads dispersed around six to 10 high-security Pakistani military bases.

Military sources say contingency plans have been reviewed over the past three days to prevent any of Pakistan's atomic weapons falling into the hands of Islamic extremists if the administration of President Pervez Musharraf appears threatened by civil unrest.

Some of the special forces are already believed to be in neighbouring Afghanistan and on alert for the mission. It is also understood that satellite surveillance of Pakistan has been stepped up to keep track of the possible movement of nuclear weapons and missile delivery systems.

According to a US Congressional report published in November, Pakistan's nuclear deterrent consists of warheads for missiles and bombs dropped from aircraft.

To maintain security, the weapons are not stored fully-assembled. Warheads, detonators and missiles are kept separately, but able to be married up "fairly quickly" in the event of a national crisis such as confrontation with India.

While the US has stated publicly its confidence that Pakistan's military has the weapons "under effective technical control", Secretary of State Condoleezza Rice admitted two years ago that if there was a radical Islamic coup, the US was "prepared to try to deal with it".

Internal security at Pakistan's nuclear storage sites is the responsibility of a 10,000-man security force commanded by a two-star general. Every member of the force is vetted with the aim of weeding out sympathisers of the Taliban and al Qaeda or anyone with extreme Islamic views.

US diplomatic and military initiatives since 2001 have concentrated on trying to ensure that pro-western commanders were in charge at the most sensitive sites.

There has also been pressure to keep Pakistan's ISI intelligence agency, thought to contain a number of high-ranking pro-Taliban supporters, out of the nuclear loop.