LONDON Pakistan's nuclear installations are so well guarded that Islamist militants behind a wave of violence in the country's heartland would find it very hard to storm them and steal material for a nuclear bomb, analysts say.
But the sophistication of recent attacks and their proximity to Pakistan's nuclear infrastructure suggest this risk, while low, remains a cause for worry.
"I don't think it is realistic any more to say there is no threat to these weapons, that they are totally safe," said Professor Shaun Gregory at Britain's University of Bradford.
Heavy conventional guarding, a blanket of secrecy, deliberate deception, the separation of warheads from missiles, and security practices adopted from the United States are all used to protect weapons and nuclear installations.
The nightmare scenario would be of militants using a suicide bombing as a diversionary tactic in order to send in a team of commandos -- similar to those who attacked the Pakistan Army's own headquarters last month in the city of Rawalpindi.
Then, and given the secrecy surrounding Pakistan's nuclear programme this would need collusion and information from inside, they would try to grab fissile material for a nuclear bomb.
"If commandos managed to penetrate a nuclear installation, that would be a very serious breach," said Sharon Squassoni, at the Carnegie Endowment for International Peace in Washington.
"It's likely they would fail, it's very likely they would fail, but that would be a bit too close for comfort."
It is a scenario the army, which guards its nuclear assets as its best defence against Pakistan's bigger neighbour India, will do everything in its power to avoid.
"If the Pakistan Army does one thing, it will be to ensure the nuclear assets stay with them," said Rahul Roy-Chaudhury at the International Institute for Strategic Studies in London.
"These are the key strategic assets of the Pakistan Army. This is what prevents India from attacking them, in their view."
Indeed so well guarded are the nuclear weapons that in the event of war, no outside power could take them out.
"The Americans, the Indians, the Israelis do not have any chance of spiriting these weapons away except with the help of the Pakistan Army," said Gregory.
The military has also long been aware of the need to keep Islamist militant sympathisers away from the nuclear weapons programme -- a concern frequently cited given close links in the past between the army and various militant groups.
General Khalid Kidwai, head of the army's Strategic Plans Division which runs the nuclear programme, conducts intensive personnel vetting modelled on U.S. personality profiling.
"He is very much in control of things," said Roy-Chaudhury. "I believe he runs a strong counter-intelligence programme. Anyone employed is closely vetted; senior officers are selected personally by him."
According to analysts and research reports, the warheads are not mated with aircraft and missile delivery systems.
Nor are they moved except in times of crisis -- as happened in 1999 and 2001/2002 during confrontations with India.
Other security measures cited by analysts, but hard to confirm, include the separation of warheads from detonators.
They also say the army is believed to have developed a rudimentary system to electronically lock its nuclear weapons, modelled on the U.S. Permissive Action Link (PAL).
Nonetheless, the ability of Islamist militants, some linked to al Qaeda, to strike apparently at will in Pakistan's heartland Punjab province, give pause for thought.
Both Rawalpindi and Islamabad, respectively the military and political capitals, have come under attack in recent weeks.
Last month's attack at army headquarters in Rawalpindi demonstrated a sophistication in tactics which could be developed further for an assault on a nuclear installation.
The gunmen, wearing army uniform, were able to break through a security gate, and take hostages.
The raid, and the drive-by fatal shooting in Islamabad of a brigadier on leave from peacekeeping duties in Sudan, also suggested militants had good intelligence on the military. Two other brigadiers were shot at in Islamabad, but survived.
According to Gregory, over the last few years militants had launched attacks outside bases believed to be involved in part of Pakistan's nuclear infrastructure -- including the manufacture and assembly of components.
They had also evolved forms of attack which could combine suicide bombers with ground assault teams. "This tactic could be used to penetrate even highly defended sites," he said.
And while Pakistan's nuclear arsenals are thought to be stored deep within the country, possibly in Sind and Baluchistan provinces, nuclear weapons production sites in Punjab, scene of much of the latest violence, could prove more vulnerable.
This would not mean the attackers would obtain a functioning nuclear bomb. And even if they were able to penetrate a nuclear production site, they would still have to manage the logistics under fire of removing the 25 kg (55 lb) of fissile material needed to make a nuclear bomb.
But they could seize material for a dirty bomb -- which causes relatively few casualties but major environmental damage -- and valuable technology while scoring a propaganda coup.
(Editing by Matthew Jones)