WASHINGTON (AP) _ he Customs Service's seizure of a speedboat with three crew members and 20 Cubans aboard Thursday demonstrates U.S. resolve to prevent a repeat of the 1980 Mariel boatlift, Attorney General Janet Reno said.

''Federal, state and local authorities, and the Florida community, all agree that it is not in the interest of the people of the United States or the people of Cuba to permit another Mariel-type exodus from Cuba,'' Reno told an evening news conference.

''The solution to emigration pressures from Cuba is rapid, fundamental and far-reaching political and economic reform,'' she said. ''Uncontrolled exodus from Cuba does nothing to address Cuba's internal problems and places large numbers of persons at risk on the high seas.''

Reno spoke several hours after Customs officials picked up the three crew members soon after the speedboat docked at a Key Largo resort in the Florida Keys. Customs officials had been tipped the Cubans would be dropped off. The three were turned over to the U.S. Border Patrol and were being held at a detention center.

''If federal law enforcement officers find probable cause to believe a vessel is being used in an attempt to bring undocumented migrants to the U.S. in violation of U.S. law, the vessel will be seized,'' Reno said.

She said that ''in appropriate cases'' steps would be taken to have the vessels forfeited to the U.S. government through administrative or judicial procedures. And, the attorney general added, ''In appropriate circumstances, individuals who violate the law will be criminally prosecuted.''

In a speech broadcast via satellite Thursday night from Havana, Cuba President Fidel Castro accused the United States of covering up the alleged hijacking of Cuban vessel and the killing of a Cuban military officer.

He cited a statement by a U.S. Coast Guard spokesman who had said that, based on interviews with the Cubans on the boat, it appeared that no one had been killed.

Castro said the body of Cuban Lt. Roberto Aguilar Reyes was found Wednesday, two days after the hijacking of the government boat.

''If we hadn't found the body, they would have continued propagandizing that the man who hijacked the boat was the man who we say was killed,'' Castro said.

U.S. officials detained a military mechanic, Leonel Macias Gonzalez, after he and 25 companions arrived at Key West, Fla., on Wednesday. Macias Gonzalez, 19, was being held at the local county jail.

Earlier Thursday, the State Department said the Coast Guard is prepared to rapidly put more ships in the Caribbean in response to any mass migration from Cuba.

Press officer David Johnson said the Coast Guard will play a central role in ''interdicting and rescuing any influx of refugees in the Straits of Florida.''

Johnson gave no details but Rep. Robert Torricelli, D-N.J., chairman of the House Foreign Affairs subcommittee on Inter-American Affairs, said Wednesday the Pentagon could quadruple its 12-vessel patrol in 36 hours if needed.

At the Pentagon, spokesman Dennis Boxx said 12 U.S. Navy ships are deployed off Haiti and an additional five are stationed in the southern Caribbean on anti-narcotics duty. It was not clear whether Boxx and Torricelli were referring to the same 12 vessels.

Boxx suggested the vessels off Haiti and in the southern Caribbean could be redeployed off Cuba if the need arises but only if they can be spared. Another possibility is using ships deployed off the U.S. East Coast, he said.

Administration officials have been updating contingency plans for coping with a mass influx of Cubans since President Fidel Castro warned of that possibility Friday. In 1980, 125,000 Cubans fled to the United States in the Mariel boat lift.

Reno told reporters earlier Thursday that U.S. officials are working with officials in Miami ''to caution Castro that he's not going to be able to do a repeat of Mariel.''

Cuba accuses the United States of putting restrictions on legal emigration while doing nothing to deter Cubans from fleeing the country without authorization.

Over the past two weeks, three Cuban vessels carrying a total of 158 passengers have been seized by Cubans seeking to flee to the United States.

The incidents have led to some violence and Cuba has asked for early discussions with U.S. officials to resolve the issue. But the Clinton administration has shown no interest.

Cuba's ambassador to Mexico said in Mexico City Thursday that his country and the United States can solve a crisis over immigration by negotiating.

If the United States would grant 20,000 visas each year under a 1984 agreement between the two countries, ''it would be an immediate solution,'' Ambassador Abelardo Curbelo Padro said at a news conference.