The administration’s effort is complicated by the fact that Pakistan is producing an unknown amount of 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  the hardest part of making a nuclear weapon  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.”

Obama administration officials said that 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 public confirmation that the arsenal is increasing  a view widely held in both 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.

The Congressional briefings have taken place in recent weeks as Pakistan has descended into further chaos and as Congress has considered proposals to spend $3 billion over the next five years to train and equip Pakistan’s military for counterinsurgency warfare. That aid would come on top of $7.5 billion in civilian assistance.

None of the proposed military assistance is directed at the nuclear program. So far, America’s aid to Pakistan’s nuclear infrastructure has been limited to a $100 million classified program to help Pakistan secure its weapons and materials from seizure by Al Qaeda, the Taliban or “insiders” with insurgent loyalties.