WASHINGTON (Reuters) -- Pakistan's nuclear facilities have come under attack from the Taliban and other groups and there is a "genuine" risk militants could seize weapons or bomb-making material, an article published in a West Point think tank newsletter said.
The Pentagon, seeking to bolster Pakistan's government in its fight against Al-Qaeda and Pakistani Taliban forces, expressed satisfaction with security at the facilities.
Defense Secretary Robert Gates and Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, are "comfortable with the security measures the Pakistani government, the Pakistani military have in place to ensure that their nuclear arsenal is safeguarded," press secretary Geoff Morrell told reporters.
The Combating Terrorism Center, which is housed at the U.S. Military Academy at West Point, published the article in the July edition of its "Sentinel" newsletter, copies of which were distributed widely on August 11.
The center said the views expressed in the article were those of the author, and not those of West Point, the Army, or the Defense Department.
Written by Shaun Gregory, director of the Pakistan Security Research Unit at the University of Bradford in Britain, the article detailed three attacks against Pakistan's nuclear facilities, and warned that sites in the country may be vulnerable to infiltration.
"The risk of the transfer of nuclear weapons, weapons components, or nuclear expertise to terrorists in Pakistan is genuine," the article said.
U.S. officials say Washington has taken steps to mitigate the risks, such as by giving Pakistan assistance in checking containers leaving from key ports for radioactive materials.
Gregory wrote that Pakistani forces guarding the facilities underwent a selection process to keep militant sympathizers out. For added protection, warhead cores are separated from their detonators, and these components are kept in underground sites.
Some 8,000 to 10,000 members of the Pakistani army's Strategic Plans Division and other intelligence agencies are involved in providing security and monitoring, he said, citing interviews with Pakistani and French officials.
"Despite these elaborate safeguards, empirical evidence points to a clear set of weaknesses and vulnerabilities in Pakistan's nuclear safety and security arrangements," Gregory wrote.
To guard against a possible Indian offensive, Pakistan located most of its nuclear weapons infrastructure in the north and west of the country, and in areas near Islamabad and Rawalpindi.
"The concern, however, is that most of Pakistan's nuclear sites are close or even within areas dominated by Pakistani Taliban militants" and Al-Qaeda, Gregory said.
He cited three attacks -- one on a nuclear missile storage facility in November 2007, one a month later on a nuclear airbase, and an August 2008 attack in which Pakistani Taliban suicide bombers blew up several entry points to an armament complex at one of Pakistan's nuclear weapons assembly sites.
A U.S. intelligence official, speaking on condition of anonymity, said the facilities described by Gregory were large and it was unclear whether the attackers knew what they contained.
"If they were after something specific, or were truly seeking entry, you'd think they might use a different tactic, one that's been employed elsewhere -- such as a bomb followed by a small-arms assault," the intelligence official said.
"Simply touching off an explosive outside the gate of a base -- with no follow-up -- doesn't get you inside. For those reasons, I wouldn't extrapolate from these incidents any kind of downgrade in the security of the Pakistani nuclear arsenal," the official added.
Pakistan is believed to have stockpiled approximately 580-800 kilograms of highly enriched uranium, sufficient amounts to build 30-50 fission bombs.
The "Bulletin of the Atomic Scientists" estimated in 2007 that the Pakistani arsenal comprised about 60 warheads.