Pakistan is unable to protect its growing atomic arsenal from the threat of Islamic extremists, according to one of the country's leading nuclear scientists.
Their security in a country known for political instability has long been a matter of concern and has taken on added urgency in recent weeks, ever since militants took control of a naval base in Karachi, holding elite troops at bay for more than 16 hours.
Professor Pervez Hoodbhoy, who teaches at universities in Lahore and Islamabad, said there was evidence that the Army had been infiltrated by extremist elements.
"We have reason to worry because the most secure installations, bases, and headquarters of the military have been successfully attacked by Islamic militants who have sympathisers within the military," he said.
"What is the proof that nuclear installations or weapon stocks would be exempt from this? My worry is not limited to nuclear arsenals because places that deal with fissile materials can also be similarly infiltrated."
Pakistan's military is reeling from a series of humiliating attacks that has led to fresh questions about whether it can protect the country from enemies – and its nuclear weapons, thought to number up to 120 warheads, from militants.
Last month, officers failed to detect a covert raid conducted by US special forces as they raided Osama bin Laden's hideaway.
Later that month, the Pakistan Taliban claimed responsibility for attacking PNS Mehran, a naval base in Karachi.
That attack suggested the "safety and security of nuclear weapons materials in Pakistan may very well be compromised," according to an article published in the Combating Terrorism Center's magazine, Sentinel, at the US military academy West Point.
"A frontal assault on nuclear weapons storage facilities, which are the most robustly defended elements of Pakistan's nuclear weapons cycle, is no longer an implausible event," wrote Shaun Gregory, director of the Pakistan Security Unit at the University of Bradford in Britain.
But Pakistan has always insisted its nuclear weapons were safe.
And last month Anders Fogh Rasmussen, Nato secretary-general, said: "I feel confident that Pakistan's nuclear arsenal is safe and well protected. But of course it is a matter of concern and we follow the situation closely."