Members of Congress have been told in confidential briefings that Pakistan is rapidly adding to its nuclear arsenal even while racked by insurgency.
The briefings raised questions on Capitol Hill about whether billions of dollars in proposed military aid might be diverted to Pakistan's nuclear program.
Adm. Mike Mullen, chairman of the Joint Chiefs of Staff , confirmed the assessment of the expanded arsenal in a one-word answer to a question Thursday in the midst of Senate testimony.
Sitting beside Defense Secretary Robert Gates, he was asked if he had seen evidence of an increase in the size of Pakistan's nuclear arsenal.
"Yes," he said quickly, adding nothing, clearly cognizant of Pakistan's sensitivity to any discussion about its nuclear strategy or security.
Inside the Obama administration, some officials say, Pakistan's drive to spend heavily on nuclear arms has been a source of growing concern, because the country is producing more nuclear material at a time when Washington is focused on trying to assure 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 an unknown amount of bomb-grade uranium and, once a series of new reactors is completed, bomb-grade plutonium for a new generation of weapons.
President Barack Obama has called for passage of a treaty that would halt all nations from producing more fissile material - the hardest part of making a nuclear weapon - but so far has said nothing in public about Pakistan's efforts.