WASHINGTON — A federal criminal investigation has uncovered evidence that Pakistan has made clandestine purchases of U.S. high-technology components for use in its nuclear-weapons program in defiance of U.S. law.

Federal authorities also said the highly specialized equipment passed through the hands of an arms dealer in Islamabad, Pakistan, named Humayun Khan, who they said has ties to Islamic militants.

Even though President Bush has been pushing for an international crackdown on such trafficking, efforts by two U.S. agencies to send investigators to Pakistan to gather more evidence have been stymied for more than a year by the State Department, according to U.S. officials knowledgeable about the case.

The impasse is part of a larger tug of war between federal agencies that enforce U.S. nonproliferation laws and policy-makers who consider Pakistan too important to embarrass after President Pervez Musharraf threw his support to the Bush administration's war on terrorism and the invasion of neighboring Afghanistan to oust Pakistan's former Taliban allies.

Bush yesterday confirmed that the U.S. is selling at least 24 F-16 fighters to Pakistan, a move a State Department official said was done out of "gratitude" for Musharraf's support.

"This is the age-old problem with Pakistan and the U.S. Other priorities always trump the United States from coming down hard on Pakistan's nuclear proliferation. And it goes back 15 to 20 years," said David Albright, director of the Washington-based Institute for Science and International Security. Albright, a former U.N. weapons inspector in Iraq and elsewhere, favors getting tougher with Pakistan.

U.S. and European officials involved in nonproliferation issues said they recently discovered evidence that Pakistan has begun a push to acquire advanced nuclear components in the black market as it tries to upgrade its 30-year-old nuclear program.

Current and former U.S. intelligence officials said the same elements of the Pakistan military that they suspect of orchestrating efforts to buy U.S.-made products also might have worked with Abdul Qadeer Khan, the so-called father of the Pakistani nuclear program who supplied nuclear weapons know-how and parts to Iran, North Korea and Libya.

U.S. law prohibits the sale of equipment that can be used in nuclear-weapons programs to Pakistan and some other countries as part of the American effort to curb nuclear proliferation.

Although the State Department last year scuttled an investigative trip to Pakistan by Commerce and Homeland Security departments investigators, several senior U.S. officials said the United States had made high-level requests of Islamabad for cooperation in the case.

They said, however, that none was made forcefully or publicly.