SAO PAULO, BRAZIL -- President Jose Sarney has affirmed that Brazil has crossed the nuclear threshhold to join the restricted club of nations capable of enriching uranium, but he promises it will not build atomic weapons.

Sarney said on Sunday that scientists working at an institute in Sao Paulo -- the existence of which until now was not acknowledged officially -- had without outside help achieved "complete domination" of the ultracentrifuge process for enrichment of uranium. This process is used to create fuel for nuclear reactors, and can be used to produce weapons-grade plutonium.

Although experts said uranium enrichment had not yet surpassed 1.2 percent, a new plant supervised by the Navy would soon achieve 20 percent enrichment in industrial quantities. Weapons-grade uranium requires 90 percent enrichment, but less pure uranium can produce plutonium.

{In Washington, the State Department said "we are as yet unaware of the details" of the development announced by Sarney "and have requested additional information from Brazilian officials." A spokesman said that while the United States considered Sarney's statement that Brazil is committed to an exclusively peaceful nuclear program to be "encouraging, any spread of unsafeguarded uranium enrichment technology is of proliferation concern."

{Leonard Spector, of the Carnegie Endowment for International Peace, said the development appeared to represent "quite a bit of progress -- more than had been anticipated" for Brazil's nuclear capability. The announcement indicated that Brazil has "figured out how to make these centrifuges work," he said. Although Brazil still appears to be far from achieving the grade of enrichment and the quantity needed for nuclear weaponry, he added, the "new capability" is "something that will have to be watched closely."}

The United States and its allies first blocked exports of ultracentrifuge enrichment technology to Brazil in the 1950s. In 1976, they vetoed West German proposals to hand over the secret as part of a multibillion-dollar accord that was to include eight nuclear power plants.

Brazil is not a signatory of the nuclear Non-Proliferation Treaty. When the military then ruling Brazil discovered that safeguards surrounding the West German agreement would prevent acquisition of weapons technology, it embarked on a secret "parallel" program, centered largely at the Institute of Nuclear and Energy Research at the University of Sao Paulo.

"Brazil's commitment to use nuclear energy exclusively for peaceful purposes is unquestionable," said Sarney, repeating his commitment to a nuclear-free Latin America as envisioned in the 1962 Treaty of Tlatelolco. Brazil signed and ratified the treaty, but in such a way that it evades responsibilities until all Latin American countries have done so. Argentina has not ratified the treaty.

"We are not going to build the bomb because we do not want the bomb," said Rex Nazareth, president of the nuclear energy authority. But the discovery last year of a supposed underground weapons testing site in the Amazon, and development of a rocket much like a ballistic missile have raised suspicions.

{An analysis published by the Congressional Research Service last month named Brazil as a country that plans "to have fairly capable satellite launch vehicles around 1990," which would give it "a capability to deliver weapons payloads to most significant targets in neighboring countries."}

Traditional rivalry between Brazil and Argentina's past military governments recently has been replaced by nuclear cooperation. Last month, Sarney visited the plant where Argentina first enriched uranium in 1983. Sarney forewarned Argentina of his announcement and the two countries may soon begin exchanging enriched nuclear material without international safeguards. Both are known to be planning nuclear-powered submarines.

Sarney's announcement that Brazil had beaten the big powers' nuclear boycott could soon result in a ban on sales of sensitive equipment. The State Department reportedly has held up the sale of a Cray supercomputer to the national oil company because of the possibility that Brazil could violate U.S. requirements and use it for a weapons program.

Sarney's anouncement was carefully timed to reinforce the idea that Brazil's military is serving the long-term interests of the nation, just as opposition parties were raising the specter of the generals hampering the transfer to full democracy.

Army Minister Gen. Leonidas Pires Goncalves recently warned a Cabinet meeting that the Army would not tolerate proposed changes in the constitution that would curtail the military's right to use force whenever it considered internal law and order to be under threat. The general also attacked proposals to trim Sarney's contingent of six uniformed Cabinet ministers to a single civilian defense minister.

Soon afterward, Sarney praised the irritated general as "one of the pillars of democratic transition." On the eve of Brazil's independence day military parade on Monday, the president announced the nuclear breakthrough in strongly nationalist terms that clearly pleased the military.