ISLAMABAD: It's a nightmare scenario: al Qaeda militants gain control of a Pakistani nuclear weapon, either through a base assault, theft or a rogue commander's cooperation, possibly in the event of hostilities with nuclear-armed neighbour India.
While most experts believe Pakistan's strategic nuclear arsenal is safe, items such as low-yield, mobile nuclear delivery systems -- called "shoot and scoot" tactical nuclear weapons -- could be highly vulnerable.
The Nasr (Hatf-9) system being developed is a short-range, surface-to-surface multi-tube ballistic missile system. With a range of 60 km (37.5 miles), it's designed for battlefield use.
Deploying small, portable nuclear weapons onto a battlefield increases the risk of things going wrong, either through miscalculation, an accident or possible infiltration by militant groups, nuclear experts say.
Here are some scenarios of how Pakistan might lose control over some of its nuclear arsenal.
Fog of (Nuclear) War
In the event of hostilities between India and Pakistan, militants could seize control of a Hatf-9 system -- essentially a rocket launcher on a truck. But could they use it?
It's necessary to understand the circumstances. In case of war between Islamabad and New Delhi, India is developing a "Cold Start" doctrine, which envisions armored battle groups quickly pushing into Pakistani territory, holding key pockets and then forcing Islamabad to the negotiating table.
The plan is to avoid antagonizing Pakistan to the point that it would retaliate with ballistic missiles against Indian cities, but Pakistani doctrine -- and the Hatf-9 system -- appears to envision using tactical nuclear weapons, possibly on its own soil, against the invaders.
But deploying tactical weapons to the battlefield means command and control has to be dispersed to individual military units.
And while Pakistan normally separates warheads and delivery systems, in times of crisis, weapons would be armed and deployed, although still "locked" by authentication codes, says Professor Shaun Gregory, director of the Pakistan Security Research Unit at the University of Bradford.
"However in a fluid battlefield context such codes will likely be released to prevent the weapons being overrun before they can be used," he told Reuters in an email. "In such a 'release delegated' state ... it's possible that terrorists could seize a functioning weapon."
Pakistan says its weapons have indigenously developed safety systems to prevent misuse, but it has never allowed outsiders to inspect these systems, Rahul Roy-Chaudhury, senior fellow for South Asia at the International Institute for Strategic Studies in London, told Reuters.
Another scenario is the "rogue commander". Militant groups have had varying degrees of success in infiltrating the Pakistani military, but it's unclear how deeply they go.
In addition to mechanical safety systems, Pakistan says its nuclear weapons are secure because of rigorous background checks and continuous monitoring of personnel for extremist sympathies.
That monitoring appears insufficient.
As far back as 2006, the United States was reporting in diplomatic cables, recently published by Wikileaks, that Pakistani pilots and flight crew were engaging in "petty sabotage" of American F-16s to prevent them being used against Taliban militants in the northwest.
The bodyguards for Punjab governor Salman Taseer were also heavily screened, but he was killed by an extremist who managed to get assigned to his security detail.
Speculation is also rife that the Taliban assaults on the PNS Mehran naval base in Karachi this month and on the Army General Headquarters in Rawalpindi in 2009 were aided by sympathisers from within the military.
While a commander going rogue is unlikely, "this is an ongoing struggle," Roy-Chaudhury said. "The expected increase in radicalisation, especially within the Pakistani army after the U.S. raid and killing of Osama bin Laden in Abbottabad ... raises additional concerns."
Rogue commanders could, in a conflict with India, hand over codes and weapons to militants or cooperate with them. The more mercenary types might simply sell them.
Seizing A Tactical Weapon From A Base
Another possibility doesn't require a fight with India. The PNS Mehran attack shows militants have developed the ability to attack and hold ground inside sensitive military bases. And while no nuclear facilities have been penetrated yet the possibility has caused great concern.
"I think the attack on PNS Mehran (and on the GHQ in 2009) show that terrorists are developing tactics which enable them to penetrate highly-secure bases and hold space within them for hours," Gregory said. "This suggests nuclear weapons security is increasingly vulnerable."
Because of their smaller size, tactical weapons are more portable. And while the warheads wouldn't be armed, the fissile materials of the cores could conceivably be extracted and used with conventional explosives to create a "dirty bomb".
Militants could attack a base, seize a warhead or its core materials and then escape. A Pakistani tactical nuclear weapon might contain as little as 15-20kg (33-44 lbs) of enriched uranium.
"Responsible Pakistani stewards of their nuclear assets have no choice but to re-evaluate their security requirements and procedures," said Michael Krepon, director of the South Asia and Space Security programs for Stimson, a Washington-based think tank for international security.