But the United States does not know where all of Pakistan’s nuclear sites are located, and its concerns have intensified in the last two weeks since the Taliban entered Buner, a district 60 miles from the capital. The spread of the insurgency has left American officials less willing to accept blanket assurances from Pakistan that the weapons are safe…

The Obama administration inherited from President Bush a multiyear, $100 million secret American program to help Pakistan build stronger physical protections around some of those facilities, and to train Pakistanis in nuclear security.

But much of that effort has now petered out, and American officials have never been permitted to see how much of the money was spent, the facilities where the weapons are kept or even a tally of how many Pakistan has produced. The facility Pakistan was supposed to build to conduct its own training exercises is running years behind schedule…

Several current officials said that they were worried that insurgents could try to provoke an incident that would prompt Pakistan to move the weapons, and perhaps use an insider with knowledge of the transportation schedule for weapons or materials to tip them off. That concern appeared to be what Secretary of State Hillary Rodham Clinton was hinting at in testimony 10 days ago before the House Appropriations Committee. Pakistan’s weapons, she noted, “are widely dispersed in the country.”