Senior Bush administration officials have asked Congress to continue U.S. aid to Pakistan even though that country may have developed a nuclear weapon, administration and congressional sources said yesterday.

By law, no U.S. aid can be given unless President Bush certifies that Pakistan does not have a nuclear weapon and that U.S. aid reduces the possibility Pakistan will develop one. U.S. aid to Pakistan totals nearly $600 million a year, making it the third largest recipient after Israel and Egypt.

"It's going to be hard for us to certify that Pakistan doesn't have a nuclear weapon {this year}," said an administration official who asked he not be identified. He said the administration had amassed "cumulative evidence" pointing to the existence of a Pakistani bomb, but he declined to provide details.

Several other officials said they understood Pakistani engineers had fabricated highly enriched uranium into a metallic form capable of being placed inside a bomb, a key step that virtually rules out any certification that no such bomb existed.

State Department deputy spokesman Richard A. Boucher at a news briefing yesterday said the question of a presidential certification is "under review." Such certifications are due Oct. 1 at the start of each fiscal year, but have often been provided late.

Several officials said waiving the certification requirement for an unspecified period would give a new Pakistani government to be elected Oct. 24 an opportunity to curtail its nuclear weapons work.

The executive branch has not previously sought an outright waiver of the 1985 nuclear nonproliferation requirement, instead issuing certification on the narrow grounds that Pakistan did not possess a nuclear explosive device. But this year, an official said, "there is ample evidence that the {bomb} program has continued unabated, and that they deserve to lose the certification."

Officials confirmed a New York Times report yesterday that all Pakistani aid would be suspended until such a certification is issued. Discussing the possibility of an outright waiver, the officials said they did not know what impact an aid cutoff would have on Pakistan's hard-pressed economy.

The waiver was raised by State Department officials with key legislators last week, sources said. Some congressional sources said a waiver proposal would be highly controversial. Congressional leaders are to discuss the issue today.

Some congressmen have said they believe Pakistan has made progress in the last year in its nuclear weapons development program. Rep. Stephen J. Solarz (D-N.Y.), chairman of the House Foreign Affairs subcommittee on Asia, wrote Bush on Sept. 19 that he believed "Pakistan's commitments to respect the various markers we set last year have not been met." If this is so, Solarz wrote, "I believe that we have no choice but to terminate all U.S. assistance to Pakistan, as required by law."

Cutting off aid would put the administration in a difficult position. Pakistan, a Moslem country, has agreed to send ground forces to the Persian Gulf, and also has played a key role in funneling U.S. aid to the anticommunist rebels in neighboring Afghanistan.

U.S. officials say the gulf turmoil could cost Pakistan $1.1 billion in rising oil costs and loss of paychecks sent home monthly by thousands of Pakistani workers in Kuwait.

Department officials say that before the Oct. 24 elections, it will be difficult to obtain required Pakistani government assurances to cease or roll back its nuclear development program.

Leonard Spector, a nonproliferation expert at the Carnegie Endowment, said a waiver would mean "we are in effect announcing to the world that Pakistan has nuclear weapons, and we just don't care."