Cuba became the first nation to reject an invitation to the 1988 Olympics in South Korea when the Caribbean nation's Olympic committee president, Manuel Gonzalez Guerra, announced yesterday that his athletes would stay home.

At a news conference in Havana, Gonzalez Guerra said International Olympic Committee President Juan Antonio Samaranch had been notified of the decision by telephone and in a letter from President Fidel Castro.

Castro's letter expressed regret, but added, "We will not be discouraged and will continue preparing for the Olympics in Barcelona in 1992, if {the IOC does not} commit the senseless act of sanctioning us for honest conduct."

The letter also said, "Moral principles . . . are more important than the emotions of the Olympic Games and the gold medals that could be obtained," according to a report from the Cuban news agency Prensa Latina.

Wrote Castro, "Even our grade school children understand that with bullets, tear gas and massive repression of the people there would not be the most healthy and honorable conditions for the Olympic Games."

The letter left open the door for a change of heart if the safety of Cuban athletes was guaranteed and North Korea was named a cohost of the Games, but close Olympic observers held out no likelihood the conditions would be met.

"The North Koreans have been offered five events, but that does not constitute co-hosting and the IOC has indicated that offer is firm," said U.S. Olympic Committee spokesman Mike Moran.

When the Soviet Union refused to send a team to the 1984 Los Angeles Summer Olympics, concern for the safety of the Soviet athletes was given as the reason.

The Cuban announcement cast a shadow over the upcoming Games in Seoul in September, but U.S. Olympic Committee President Robert Helmick said in Atlanta the decision was no surprise to him and would not significantly diminish the Games.

"The significance of these Games is that 160 nations are participating, not that a few don't," Helmick said.

The record number of participants already signed up includes the Soviet Union and China, allies of communist Cuba, both of which joined the Cubans in boycotting the 1984 Games in Los Angeles.

Only six of 166 invited nations remain uncommitted to Seoul, Helmick said, with two days left to make their intentions known. They are North Korea, Albania, Ethiopia, Nicaragua, Seychelles and Syria.

Yesterday, Vietnam and Czechoslovakia, Madagascar and Tanzania said they would attend.

The previous record participation was 140 nations in 1984, despite the Soviet-led boycott of the Los Angeles Games.

The Games will begin Sept. 17 and will continue through Oct. 2.

Cuba's decision could jeopardize the island nation's chances to participate in future Olympics. IOC officials next month will consider legislation under which boycotting nations would be banned from succeeding Olympics, which could mean the Cubans would be barred in 1992.

But USOC executive board member John Mosher, who has closely watched the boycott issue, said the Cubans may be spared any reprisals because, "This really isn't a boycott. Any nation has the right not to attend." Mosher said a boycott would involve organizing a refusal to attend by several nations, which hasn't happened.

Helmick expressed regret for the Cuban athletes. "I'm very disappointed for them," he said. "A whole generation will be denied the opportunity to participate."

Helmick said the Cubans would be missed in the competition, particularly in boxing and baseball, where they have top competitors.

Yesterday's decision will have no apparent effect on the upcoming Winter Games next month in Calgary, Canada, in which Cuba never intended to participate. North Korea is planning to send three athletes to Calgary, according to USOC officials.

When the Soviet Union announced earlier this week its intention to attend the Seoul Games, it had been hoped this would be the first boycott-free Olympics since 1972. A group of African nations boycotted the 1976 Montreal Games, the United States boycotted the 1980 Moscow Games and the Soviets and their allies stayed out of the 1984 Los Angeles Olympics.