The U.S. ambassador to Pakistan has said publicly for the first time that Pakistan's new leaders must prove they have neither an assembled nuclear weapon nor its components if they want a resumption of U.S. aid.

The view expressed by Ambassador Robert Oakley in a recent letter to The Nation, a leading Pakistani newspaper, appears to impose a stiffer requirement than the United States has publicly demanded in previous bargaining over Islamabad's nuclear weapons program.

Annual U.S. economic and military aid totaling roughly $580 million was halted on Oct. 1 when President Bush declined to certify that Pakistan has no "nuclear explosive device." Congress has required such a certification since 1985 as a condition of foreign assistance to Islamabad.

Pakistani leaders have promised in the past to meet the U.S. requirement by keeping the components of any nuclear bomb separated, and the State Department has reserved the right to accept or reject such assurances.

Some legislators had challenged this position as allowing unwarranted executive-branch discretion. "In the 1950s, the United States repeatedly stored its nuclear weapons in separate parts for safety purposes, yet who claims that the United States was not a nuclear weapon state in that period?" said Sen. John Glenn (D-Ohio), chairman of the Senate Committee on Governmental Affairs.

But Oakley, in what several U.S. and congressional officials said was an unusually explicit and tough statement of U.S. views, stated in his letter published yesterday that the executive and legislative branches were agreed that "the definition of possession . . . applies to components of a nuclear device, not only to an assembled device."

Pakistani President Ghulam Ishaq Khan has pledged since his election on Oct. 24 to resist bowing to U.S. demands to halt the nuclear weapons progam, a pledge that remains highly popular in his country. "If America is not prepared to extend aid to Pakistan of their own volition and free will, then there is nothing we would like them to do for us," the Reuter news agency quoted him as saying recently.

Several informed sources, who declined to be identified, said Bush's decision not to accept routine Pakistani assurances was based on two key developments in the nuclear weapons program during a period of heightened tensions last spring between Pakistan and India over the disputed state of Jammu and Kashmir.

The sources said that at a plant in Kahuta, roughly 25 miles from Islamabad, Pakistan had evidently resumed its enrichment of uranium to the quality required for nuclear weapons, after earlier promising that such activities would not be resumed. Pakistan also took various steps that were interpreted by the United States as potential preparations for the deployment of nuclear bombs aboard U.S.-made F-16 fighter aircraft, the sources said on condition they not be identified.

The Bush administration's fear of a major India-Pakistan conflict over Kashmir, combined with its uncertainty about the import of the Pakistani nuclear weapons moves, caused it to issue various private warnings and dispatch a high-level delegation to the region led by deputy national security adviser Robert M. Gates.

U.S. officials said Pakistani assurances last month of peaceful aims for the nuclear program did not satisfy the administration's concerns.