As the insurgency of the Taliban and Al Qaeda spreads in Pak, senior American officials say they are increasingly concerned about new vulnerabilities for Pakistans nuclear arsenal, including the potential for militants to snatch a weapon in transport or to insert sympathizers into laboratories or fuel-production facilities. The officials emphasized that there was no reason to believe that the arsenal, most of which is south of the capital, Islamabad, faced an imminent threat. President Obama said last week that he remained confident that keeping the countrys nuclear infrastructure secure was the top priority of Pakistans armed forces. But the United States does not know where all of Pakistans 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. Pakistani officials have continued to deflect American requests for more details about the location and security of the countrys nuclear sites, the officials said. Some of the Pakistani reluctance, they said, stemmed from longstanding concern that the United States might be tempted to seize or destroy Pakistans arsenal if the insurgency appeared about to engulf areas near Pakistans nuclear sites. But they said the most senior American and Pakistani officials had not yet engaged on the issue, a process that may begin this week, with President Asif Ali Zardari scheduled to visit Mr. Obama in Washington on Wednesday. We are largely relying on assurances, the same assurances we have been hearing for years, said one senior official who was involved in the dialogue with Pak during the Bush years, and remains involved today. The worse things get, the more strongly they hew to the line, 'Dont worry, weve got it under control. In public, the administration has only hinted at those concerns, repeating the formulation that the Bush administration used: that it has faith in the Pakistani Army. Im confident that we can make sure that Pakistans nuclear arsenal is secure, Mr. Obama said Wednesday, primarily, initially, because the Pakistani Army, I think, recognizes the hazards of those weapons falling into the wrong hands. He added: Weve got strong military-to-military consultation and cooperation. But that cooperation, according to officials who would not speak for attribution because of the sensitivity surrounding the exchanges between Washington and Islamabad, has been sharply limited when the subject has turned to the vulnerabilities in the Pakistani nuclear infrastructure. 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. Administration officials would not say if the subject would be raised during Mr. Zardaris first meeting with Mr. Obama. But even if Mr. Obama raises the subject, it is not clear how fruitful the conversation might be. Mr. Zardari heads the countrys National Command Authority, the mix of political, military and intelligence leaders responsible for its arsenal of 60 to 100 nuclear weapons. But in reality, his command and control over the weapons are considered tenuous at best; that power lies primarily in the hands of the army chief of staff, Gen. Ashfaq Parvez Kayani, the former director of Inter-Services Intelligence, the countrys intelligence agency. For years the Pakistanis have waved away the recurring American concerns, with the head of nuclear security for the country, Gen. Khalid Kidwai, dismissing them as overblown rhetoric. Americans who are experts on the Pakistani system worry about what they do not know. For years I was concerned about the weapons materials in Pakistan, the materials in the laboratories, said Rolf Mowatt-Larssen, who ran the Energy Departments intelligence unit until January, and before that was a senior C.I.A. officer sent to Pakistan to determine whether nuclear technology had been passed to Osama bin Laden. Im still worried about that, but with what were seeing, Im growing more concerned about something going missing in transport, said Mr. Mowatt-Larssen, who is now at Harvards Kennedy School of Government. 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. Pakistans weapons, she noted, are widely dispersed in the country. Theres not a central location, as you know, she added. Theyve adopted a policy of dispersing their nuclear weapons and facilities. She went on to describe a potential situation in which a confrontation with India could prompt a Pakistani response, though she did not go as far as saying that such a response could include moving weapons toward India which American officials believed happened in 2002. Other experts note that even as Pakistan faces instability, it is producing more plutonium for new weapons, and building more production reactors. David Albright and Paul Brannan of the Institute for Science and International Security wrote in a recent report documenting the progress of those facilities, In the current climate, with Pakistans leadership under duress from daily acts of violence by insurgent Taliban forces and organized political opposition, the security of any nuclear material produced in these reactors is in question. The Pakistanis, not surprisingly, dismiss those fears as American and Indian paranoia, intended to dissuade them from nuclear modernization. But the governments credibility is still colored by the fact that it used equal vehemence to denounce as fabrications the reports that Abdul Qadeer Khan, one of the architects of Pakistans race for the nuclear bomb, had sold nuclear technology on the black market. In the end, those reports turned out to be true.