After Karachi: Is Pakistanâ€™s nuclear arsenal safe? The assault by Pakistani militants on the naval air station at Mehran in Karachi represents a highly sophisticated attack against an important military installation. The base is home to Pakistan's US-supplied Orion P3-C maritime patrol aircraft. At least two of the aircraft were destroyed. A few US contractors - as well as a small number of Chinese engineers - were also at the base. Their presence highlights the peculiar split nature of Pakistan's military alliances. The attack, audacious by the standards of Pakistan's Taliban, raises all sorts of awkward questions about security at the facility. Did standards just slip? Were the militants underestimated? And what does it imply for security at other key installations, not least those associated with Pakistan's nuclear deterrent? Islamabad's sensitivity Details on Pakistan's nuclear forces are hard to confirm. The country is estimated to have a nuclear arsenal of between 70 and 90 warheads, and it is busily modernising its capabilities across the board. This includes new weapons systems to deliver the warheads along with new nuclear facilities to provide the crucial nuclear materials needed to expand its arsenal. Little is known about the exact location of Pakistan's nuclear weapons. US officials have said that they are widely dispersed, and experts believe that they are stored unassembled with the nuclear cores separate from the rest of the weapons system. Washington has sought to assist Pakistan with various technical measures to safeguard its weapons. However, such efforts have not gone as far as they might, tending to founder on Pakistan's sensitivity about divulging details of its nuclear arsenal. Indeed, it is striking that in the wake of the US attack that killed Osama Bin Laden there was considerable comment in Pakistan about the ease with which the US pulled off the operation. One of the key concerns was that if the US special forces could do this, might they not be able to swoop down and seize Pakistan's nuclear weapons as well? The suggestion is probably fanciful, but it speaks volumes about the underlying suspicions that exist between Islamabad and Washington. Reasons to worry Until recently US officials took Pakistan's pronouncements of the safety of its nuclear arsenal pretty much at face value - at least in public. But as violence inside the country has grown, so too have worries in Washington. Last May, in an interview in the specialist magazine Arms Control Today, Gary Samore, a senior White House official who oversees non-proliferation matters, acknowledged that "the Pakistani government takes the nuclear security threat very seriously and they've put a lot of resources into making sure that their nuclear facilities and materials and weapons are well secured". However, he made his concerns crystal clear: "What I worry about is that, in the context of broader tensions and problems within Pakistani society and polity... even the best nuclear security measures might break down." One route to improving nuclear security is clearly on the diplomatic front through arms control; putting a ceiling on arsenals and halting the production of fissile material. But neither India nor Pakistan seems willing to make much progress here. Of course if India is considered, then China, too, has to be brought into the nuclear equation. Efforts to maintain the physical security of Pakistan's nuclear arsenal have been impressive up to now. But there are several reasons to worry: most of Pakistan's nuclear sites are close to or even within areas dominated by Pakistani Taliban militants. When they were first constructed, the imperative was to keep them away from the border with India to prevent them being overrun in any future conflict analysts believe that there have already been attacks on facilities housing elements of Pakistan's nuclear programme, including one against a nuclear storage facility in Sargodha in November 2007 and another, in August 2008, against the access points to the Wah cantonment, considered to be one of Pakistan's main nuclear weapons assembly sites despite elaborate efforts to screen personnel involved in the nuclear programme and the security force that guards it, there is always the danger of infiltration by people with extremist views some analysts even question the long-term reliability of key elements of the Pakistan military. For those who worry about the security of Pakistan's nuclear installations, the attack on the naval air station in Karachi is yet one more reminder that the militants inside the country are getting ever bolder.