President Reagan said yesterday the United States would try to prevent the spread of nuclear weapons by becoming a more reliable supplier of nuclear fuel and technology for peaceful purposes to countries that share the goals of nonproliferation while reducing the motivation of other nations to acquire atomic weapons.

Becoming a reliable supplier, the president indicated, was the best way for the United States to retain influence abroad on nonproliferation issues.

At the same time, halting the spread of atomic arms is not just a matter of controls on fuel for nuclear reactors. Countries have legitimate security concerns that must be understood, Reagan said.

Among the "tools" available to ease such concerns, officials acknowledged, would be a willingness to supply conventional arms when it would counter the motivation to acquire atomic weapons.

"In the final analysis," the president said in a formal announcement of his nuclear nonproliferation policy, "the success of our efforts depends on our ability to improve regional and global stability and reduce those motivations that can drive countries toward nuclear explosives."

The president's views were contained in a seven-point "policy framework" whose contents were unofficially disclosed last week. The policy is discussed only in the broadest terms with no specifics as to how it will be implemented.

Administration officials, who briefed reporters under rules that do not permit identifying them, repeatedly declined to answer questions on how the United States would deal with countries such as Pakistan, which is suspected of building nuclear weapons, and Brazil. Neither country has signed the 1970 Nuclear Nonproliferation Treaty.

These officials also billed the policy as marking an important shift in emphasis from the Carter administration in terms of both restoring America as a reliable supplier and in the focus on regional stability as the way to stunt atomic weapons ambitions.

President Carter had made nonproliferation a major part of his foreign policy, placing strict controls on nuclear exports and pressure on allies to do the same. This produced mixed results and Reagan yesterday said many of those friends and allies "during recent years lost confidence in the ability of our nation to recognize their needs."

In another shift, Reagan said "the administration will also not inhibit or set back civil reprocessing and breeder reactor development abroad in nations with advanced nuclear power programs where it does not constitute a proliferation risk." Breeder reactors and fuel reprocessors can produce weapons-grade plutonium.

As explained by officials here, U.S. policy is to leave such decisions in the hands of the individual countries, to insist on stringent safeguards, and not to interfere when the breeder or reprocessor appears to be a "natural development" for a country already into nuclear power. Asked what the United States would do when those conditions were not present, the officials said they would try to make sure those countries understand the risks of proliferating atomic weapons.

Rep. Edward J. Markey (D-Mass.), chairman of the House Interior oversight and investigations subcommittee, charged yesterday that Reagan's announced policy was "dangerous and contradictory" and "signals a return to nuclear boosterism."

Markey said, "For the president to endorse the plutonium breeder reactor, plutonium reprocessing and the sale of nuclear technology throughout the world constitutes a policy that is certain to advance rather than restrict the spread of nuclear weaponry."

Despite its vagueness, yesterday's pronouncement may also do the president some good in the sense that it comes on the heels of a major address on arms control policy by Secretary of State Alexander M. Haig Jr. and provides some grounds to counter critics who charge the administration has no foreign policy. It also comes just before the seven-nation economic summit meeting opens in Ottawa, where nonproliferation will be on the agenda.