The United States has a detailed plan for infiltrating Pakistan and securing its mobile arsenal of nuclear warheads if it appears the country is about to fall under the control of the Taliban, Al Qaeda or other Islamic extremists.

American intelligence sources say the operation would be conducted by Joint Special Operations Command, the super-secret commando unit headquartered at Fort Bragg, N.C.

JSOC is the military’s chief terrorists hunting squad and has units now operating in Afghanistan on Pakistan’s western border. But a secondary mission is to secure foreign nuclear arsenals — a role for which JSOC operatives have trained in Nevada.

The mission has taken on added importance in recent months, as Islamic extremists have taken territory close to the capital of Islamabad and could destabilize Pakistan’s shaky democracy.

"We have plans to secure them ourselves if things get out of hand," said a U.S. intelligence source who has deployed to Afghanistan. "That is a big secondary mission for JSOC in Afghanistan."

The source said JSOC has been updating its mission plan for the day President Obama gives the order to infiltrate Pakistan.

"Small units could seize them, disable them and then centralize them in a secure location," the source said.