Pakistan's nuclear facilities have been attacked at least three times by home-grown extremists in little-reported incidents over the last two years, according to security experts, reports the Times of India.

The incidents include an attack on a nuclear missile storage facility at Sargodha on Nov. 1, 2007, and a homicide bombing at the nuclear airbase at Kamra on Dec. 10, 2007, as tracked by Shaun Gregory, director of the Pakistan Security Research Unit at the University of Bradford in the UK.

But Bradford also noted a much more considerable raid by the Pakistani Taliban on Aug. 20, 2008, when homicide bombers blew up several entry points to a main armament complex at the country's main nuclear facility, the Wah Cantonment Ordnance Complex, according to the paper.

Pakistan insists that its nuclear weapons are secure and that there is no chance of their falling into the hands of extremists or terrorists.

But these homegrown attacks have occurred even as Pakistan has taken steps to safeguard its stockpile against potential strikes, Gregory writes in the July issue of West Point's Combating Terrorism Center Sentinel.

Gregory's paper, "The Terrorist Threat to Pakistan's Nuclear Weapons," argues that Pakistan's nuclear infrastructure is designed to ward off external attacks from countries like India adn the Unites States, and asks whether the geographic location of Pakistan's principle nuclear weapons infrastructure — which is mainly in areas dominated by Al Qaeda and the Taliban — makes it more vulnerable to internal attacks.

News of the attacks has surprised even terrorism experts.

"He points out something that was news to me, which is that a series of attacks on Pakistan's nuclear weapons facilities have already happened," terror expert Peter Bergen told the Times of India.

Click here for more on this story from the Times of India.