By THOM SHANKER and DAVID E.SANGER


WASHINGTON - Members of the United States Congress have been told in confidential briefings that Pakistan is rapidly adding to its nuclear arsenal even while racked by insurgency, raising questions on Capitol Hill about whether billions of dollars in proposed military aid might be diverted to Pakistan’s nuclear program.

Admiral Mike Mullen, the chairman of the Joint Chiefs of Staff, confirmed the assessment in a one-word answer to a question during lengthy Senate testimony in May. Sitting beside Defense Secretary Robert M.Gates, he was asked whether he had seen evidence of an increase in the size of the Pakistani nuclear arsenal.

“Yes,” he said quickly, adding nothing, clearly cognizant of Pakistan’s sensitivity to any discussion about the country’s nuclear strategy or security.

Inside the Obama administration, officials say, Pakistan’s drive to spend heavily on new nuclear arms has been a source of growing concern, because the country is producing more nuclear material at a time when Washington is increasingly focused on ensuring the security of an arsenal of 80 to 100 weapons so they will never fall into the hands of Islamic insurgents.

The administration’s effort is complicated by the fact that Pakistan is producing new bomb-grade uranium and, once a series of new reactors is completed, bomb-grade plutonium for a new generation of weapons. President Obama has called for passage of a treaty that would stop all nations from producing more fissile material but so far has said nothing in public about Pakistan’s activities.

Bruce Riedel, the Brookings Institution scholar who served as the co-author of Mr.Obama’s review of Afghanistan- Pakistan strategy, reflected the administration’s concern in a recent interview, saying that Pakistan “has more terrorists per square mile than anyplace else on earth, and it has a nuclear weapons program that is growing faster than anyplace else on earth.”

Administration officials said they had communicated to Congress that their intent was to assure that military aid to Pakistan was directed toward counterterrorism and not diverted. But Admiral Mullen’s confirmation that the arsenal is increasing, a view widely held in classified and unclassified analyses, seems certain to aggravate Congress’s discomfort.

Whether that discomfort might result in a delay or reduction in aid to Pakistan is still unclear. But the billions in new proposed American aid, officials acknowledge, could free other money for Pakistan’s nuclear infrastructure.

The dimensions of the Pakistani buildup are not fully understood. “We see them scaling up their centrifuge facilities,” said David Albright, the president of the Institute for Science and International Security, which has been monitoring Pakistan’s efforts to buy materials on the black market, and analyzing satellite photographs of two new plutonium reactors less than 160 kilometers from where Pakistani forces are currently fighting the Taliban.

“The Bush administration turned a blind eye to how this is being ramped up,” he said. “And of course, with enough pressure, all this could be preventable.”

As a matter of diplomacy, however, the buildup presents Mr.Obama with a potential conflict between two national security priorities. One is to win passage of a global agreement to stop production of fissile material. Pakistan has never agreed to any limits and is one of three countries, along with India and Israel, that never signed the Nuclear Nonproliferation Treaty.

Yet the other imperative is a huge infusion of financial assistance into Afghanistan and Pakistan.

A spokesman for the Pakistani government warned against linking American financial assistance to Pakistan’s weapons program. “Conditions or sanctions on this issue did not work in the past, and this will not send a positive message to the people of Pakistan,” said the official, who spoke on condition of anonymity because his country’s nuclear program is classified.