WASHINGTON (AP) _ President Reagan acted Friday to ensure continued U.S. aid to Pakistan despite concerns about that country's possible involvement in the smuggling of nuclear-weapons materials from the United States.

The White House press office said Reagan sent to Congress papers invoking a legal provision waiving a required cutoff in aid to Pakistan, which was believed involved in an attempt to smuggle restricted materials from the United States last year.

Reagan's action, which followed a presidential certification to Congress that Pakistan did not have a nuclear explosive device, assures nonstop U.S. military and economic assistance to that country.

The move allows continued disbursement of $480 million in military and economic assistance in the current fiscal year, which ends Sept. 30, and more than $4 billion over six years.

''This waiver action was based on the recognition that disrupting one of the pillars of the U.S. relationship with Pakistan (the aid program) would be counterproductive for the strategic interests of the United States, destabilizing for South Asia and unlikely to achieve the non-proliferation objectives sought by the (congressional) sponsors,'' a press office statement said.

Congress suspended aid to Pakistan last September but voted to resume it last month, in large part because of that nation's support of guerrillas battling the Soviet-backed government of neighboring Afghanistan.

Concern about Pakistan's activities in the nuclear weapons development area were heightened when Arshad Z. Parvez, a Pakistani-born Canadian citizen, was convicted in Philadelphia last month of conspiring to export from the United States special steel used to construct uranium enrichment plants.

In a little-noticed development last month, the president met a legal requirement by certifying to Congress that Pakistan does not have a nuclear explosive.

The Parvez conviction prompted Reagan to invoke a waiver clause in a 1985 law, sponsored by Rep. Steven Solarz, D-N.Y., that requires a cutoff of U.S. aid to any nation believed to be exporting from the United States restricted materials used in the production of nuclear weapons.

Solarz, chairman of the House Foreign Affairs subcommittee on Asia, has indicated he plans to hold hearings this spring on Pakistan's nuclear development program.

The White House statement said that ''despite these (U.S.-Pakistani) problem areas, there are crucial non-proliferation criteria which Pakistan cointinues to honor. The United States will insist on the maintenance of these retraints even as we work with Pakistan on progress in the areas of concern.''

The statement said Reagan's action was ''preceded by months of extensive consultations with Congress.''

''The government of Pakistan has pledged that procedures will be tightened to ensure an end to procurement activities in the United States,'' it said. ''We will continue to monitor procurement activities in this country to ensure compliance with Pakistan's new procedures.''

It said ''there is no diminution in the president's commitment to restraining the spread of nuclear weapons in the Indian subcontinent or elsewhere.''

''We will continue to urge Pakistan and India to discuss measures which might be taken to reduce the threat of a nuclear arms race in South Asia,'' the statement said.

India in 1974 exploded what its leaders called a ''peaceful nuclear device,'' but it is not believed to have deployed atomic weapons, observers inside and outside government say.