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Pakistan is rapidly expanding its nuclear arsenal, raising questions in Congress whether billions of dollars in proposed military aid to the war-torn country could be diverted to its nuclear program, The New York Times reported on Monday.

According to the report, members of Congress have been told in confidential briefings about Pakistan's nuclear drive, Mike Mullen, the chairman of the Joint Chiefs of Staff, confirmed in Senate testimony with a single-word answer: "yes."

Pakistan's effort to build new atomic weapons has been a source of growing concern in Washington, because the country is producing more nuclear material at a time when the U.S. is increasingly focused on trying to assure the security of Pakistan's 80 to 100 weapons which it fears could fall into the hands of Islamic militants, the report said.

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The administration's effort is complicated by the fact that Pakistan is producing an unknown amount of new weapons-grade uranium and, once a series of new reactors is completed, weapons-grade plutonium for a new generation of arms, the paper added.

President Obama has called for passage of a treaty that would stop all nations from producing more fissile material.

Obama 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, The Times noted.

But Admiral Mullen's confirmation that the arsenal is expanding seems certain to aggravate the level of discomfort in Congress, the report said.

The briefings have taken place as Congress has considered proposals to spend $3 billion over the next five years to train and equip Pakistan's military for counterinsurgency warfare, the paper pointed out. The aid would come in addition to $7.5 billion in civilian assistance.