High-level talks between the United States and Brazil on nuclear energy issues ended abruptly here early this morning with both sides seeming as far apart as ever.

An American mission headed by Deputy Secretary of State Warren Christopher, the No. 2 man in the State Department, flew back to Washington from Brazilia before dawn, after just one day of negotiations. No Brazilian official accompanied the high-ranking U.S. delegation to the airport.

The talks were shrouded in secrecy, and no one from either country would say publicly just what was discussed or how the negotiations went.

The only official statement was a terse communique, precisely 25 words in its Portuguese translation, saying that both sides "exchanged opinions on nuclear matters and energy needs" and each "will consider the position stated by the other." No date was set for further discussions.

A high-ranking Brazilian official expressed the fear, after the talks ended, that Washington might apply direct economic pressure to his country to force it to change its position.

While Brazil gets little direct economic aid from the United States, it is a heavy borrower from international lending institutions, where the United States influence could be decisive.

The talks' sudden end came as a surprise. While neither side had actually said that they would take more than one day, the general expectation here was that negotiations would continue for two or three days.

Unofficial reports indicated that Brazil refused U.S. proposals that it allow the United States to enrich the uranium for its reactors or put the enrichment process under some kind of international control.

Brazil insists that either course would leave it basically dependent on outside sources for its energy needs, while the United States' concern is that Brazil - a conspicuous nonsigner of the nuclear nonproliferation agreement - may gain the ability to produce nuclear weapons.

The U.S.-Brazil talks result from President Carter's express opposition to the spread of nuclear technology.

Under its controversial pact with West Germany, Brazil is to acquire the know-how for enriching its own uranium for use as fuel in electricity-producing atomic reactors. But this know-how, experts say, will also give Brazil the capability to make plutonium, a key ingredient in nuclear weapons.

Carter has said furthermore that he will try to exert diplomatic pressure to get countries that have not signed the nuclear nonproliferation treaty to do so. Brazil feels that the treaty discriminates against developing countries.

Brazilian government spokesmen have said repeatedly that this country will not let the United States strongarm it into backing off from its agreement with Germany or into signing the nonproliferation treaty against its will.

Brazil has said, however, that it is willing to discuss "global aspects" of the nuclear problem - apparently in an effort to show the rest of the world it intends to act like a mature, responsible nation.

U.S. Secretary of State Cyrus Vance told Congress this week that American foreign aid should be conditional to the degree to which countries that receive it toe the Carter line on nuclear development. Such a policy would have little effect on Brazil, however, because this country - now gets by with almost no American developmental assistance.

Vice President Mondale, while in West Germany in January, publicly pressed the Germans to curb their sale of nuclear technology.

American and West German officials alike were vague on whether the move toward curbs could affect the Brazilian deal retroactively, although West German Chancellor Helmut Schmidt stressed that his country had always kept its word and would continue to do so.

The American pressure, and indications that Bonn may be looking for a way to alter the agreement, have caused sharp concern here.

Last week West German officials let it be known that Bonn had delayed issuing some export licenses needed for the Brazilian deal in response to American wishes.