President Reagan quietly certified in mid-December that Pakistan "does not possess a nuclear explosive device" and is expected to accept a State Department recommendation that U.S. aid continue despite Pakistan's likely involvement in a recent plot to smuggle nuclear-related materials out of the United States.

White House acceptance of State's recommendation would clear the way for the disbursal of $480 million in U.S. military and economic assistance to Pakistan this fiscal year. State Department and Pentagon officials were in Pakistan earlier this week to discuss details of the aid package, including a Pakistani request for several Airborne Warning and Control System (AWACS) surveillance aircraft.

Congress last month approved the first portion of a new, six-year $4.02 billion aid package for Pakistan after a long, divisive debate over whether and how the United States should use its influence to head off a nuclear arms race between Pakistan and India. In the end, it agreed to leave the decision up to the president, as had been done previously.

Pakistani fears that India is developing a nuclear force were heightened last week when the Soviet Union took the unprecedented step of leasing a nuclear-powered submarine armed with conventional weapons to India. U.S. experts said they believe it is the first time any of the nuclear powers have provided a Third World country with such a vessel.

One U.S. official said the administration lacks details about the leasing arrangement or the submarine but said he was "pretty sure" it was a "fairly obsolete" submarine.

The Soviet move "clearly does introduce a new level of technology in the India-Pakistan competition and gives India an edge that might be substantial," he added.

He said the administration would "probably" take up the issue with the Soviet Union since the United States was opposed to the sale or lease of nuclear reactors to a country, such as India, that does not have all its nuclear facilities under international safeguards.

Reagan's letter, mandated by Congress, was sent to congressional leaders Dec. 17 but not released publicly. One White House official, unaware of it, first asserted yesterday that it had not been issued.

Reagan wrote that based "on the evidence available and on the statutory standard, I have concluded that Pakistan does not possess a nuclear explosive device.

"The proposed United States assistance program for Pakistan remains extremely important in reducing the risk that Pakistan will develop and ultimately possess such a device. I am convinced that our security relationship and assistance program are the most effective means available to us for dissuading Pakistan from acquiring nuclear explosive devices."

Reagan's assessment comes despite U.S. intelligence reports that Pakistan has been producing weapons-grade enriched uranium at its Kahuta plant -- contrary to personal assurances to Reagan it is not -- and other evidence that it continues to seek materials necessary for building a bomb.

A study published yesterday on the incipient Pakistan-Indian nuclear arms race by the Carnegie Endowment for International Peace said U.S. officials had concluded that Pakistan has the material to build a nuclear weapon but "has chosen not to build some components and thus would probably require at least a number of weeks to deploy its first atomic bomb."

The same study said there is "little doubt" that India, which tested a device in 1974, could build and deploy a number of nuclear weapons "in a matter of weeks or months."

The administration has been far more concerned recently with bolstering Pakistan politically and militarily than with using the uncertain leverage of U.S. aid to pressure it into ending its nuclear weapons program. This is because of its essential role in U.S. efforts to force the Soviet Union to withdraw its troops from neighboring Afghanistan.

Under legislation sponsored by Rep. Stephen J. Solarz (D-N.Y.), the president also is required to determine whether cessation of U.S. aid would be "seriously prejudicial" and jeopardize U.S. security interests, even if the Pakistani government has sought to export material from the U.S. for production of a nuclear bomb.

The State Department has told the White House it has concluded the Pakistani government was probably involved in the case of Arshad Z. Parvez, a Canadian of Pakistani origin convicted last month in Philadelphia of seeking to export special steel used in building uranium enrichment plants.

Staff Writers Bill McAllister and Molly Moore contributed to this report.