MOSCOW, JAN. 11 -- The Soviet Union announced today it will take part in the 1988 Summer Olympics in Seoul, breaking ranks with its North Korean allies and ending a 12-year interruption in U.S.-Soviet competition in the prestigious quadrennial sports event.

{North Korea's Olympic committee said early Tuesday that the country will not attend the games, special correspondent Peter Maass reported from Seoul. The statement said efforts would continue for North Korea to cohost the games, a proposal rejected by the International Olympic Committee and by South Korea.}

Marat Gramov, chairman of the Soviet Sports Committee, told a press conference that Moscow had formally accepted the invitation of the International Olympic Committee, quelling any fears that the Olympics would be marred once again by a superpower boycott.

The United States led a boycott of the 1980 Moscow Olympics because of the Soviet invasion of Afghanistan.

In a move widely perceived as retaliation, the Soviet Union stayed home during the 1984 Los Angeles Olympics, citing concern for its athletes' safety.

Gramov, who broke the news of the Soviets' 1984 boycott, said today that the cycle of boycotts was "not the best page in the history of the Olympic Games."

The boycotts reflected a decline in the U.S.-Soviet relationship, which took its first plunge after the 1979 Afghan intervention and sank further during the first term of the Reagan administration. This year's resumption of the traditional athletic rivalry mirrors the upbeat mood in Moscow-Washington relations since the December meeting between President Reagan and Soviet leader Mikhail Gorbachev.

The Soviets' acceptance, delivered six days before the International Olympic Committee's deadline, followed a positive response from other East Bloc countries, including East Germany, Hungary, Poland, Romania and Bulgaria. The Mongolian Olympic Committee announced its participation in the games within hours of Moscow's acceptance. China also is expected to take part.

While today's statement offered support for communist North Korea's effort to cohost the Olympics, Gramov indicated that Pyongyang is now on its own.

"We made our decision about an hour ago," Gramov said. "As for the Democratic People's Republic of Korea, it should take its own decision in this respect."

The 1988 Olympics, which begin next month in Calgary, Canada, with winter sports, may become the first in 16 years to go ahead without a politically motivated boycott. In 1976, 23 African nations refused to send their athletes to the games in Montreal in a dispute over South Africa.

The Winter Olympics have never been boycotted.

This year, an estimated 153 countries are expected to participate at the Summer Olympics, exceeding the 140 participants in Los Angeles. The Soviets said today they will send a team of 520 athletes to compete in all of the summer events. The team will train in the Soviet Far East to help in adjusting to the South Korean climate.

Athletes from the Soviet Union, the United States and their respective allies complained they were the main losers in 1980 and 1984, having missed the chance to compete against each other. Since the two superpowers have dominated the games in the postwar period, many felt the boycotts took the edge off the competition.

Moscow and 14 of its allies cited security as the main reason for the boycott of the Los Angeles games, but Gramov said the Soviet delegation has been assured that their athletes will be protected in Seoul.

"The South Korean side has given written guarantees that Soviet athletes will be provided with all necessary conditions, including security," he said. Soviet teams participating in pregames competitions in Seoul were satisfied with the "technical state" of the sites but said the hosts' organizational skills and refereeing need improvement.

After deciding not to attend the Los Angeles games, the Soviet Union let loose a barrage of criticism against them in the official press here, accusing crowds of "jingoism," the performances of being "lackluster" and the scheduling of being warped to suit American television.

For all the intensity of the rhetoric, the Los Angeles games were nonevents here, since none was shown on television. Sports writers had to journey to the Soviet republic of Estonia, on the Baltic coast, to watch what they could on Finnish television.

To compensate Soviet and other East Bloc athletes, Moscow held the so-called Friendship 84 games, which it carefully denied were an alternative to the Olympics, something banned by the Olympic charter.

Special correspondent Maass added from Seoul:

The North Korean Olympic Committee's statement, read over domestic radio and monitored by the U.S. government-sponsored Foreign Broadcast Information Service, said, "We will not participate in the Olympic Games which South Korea attempts to host unilaterally . . . . However, we will in the future make every effort continuously to realize the cohosting of the Olympic Games."

Official sources in Seoul described the offer for more talks as a ploy to shift the blame for the boycott away from Pyongyang and onto South Korea.

Governmental and diplomatic officials here once again expressed concern that the unpredictable regime of North Korean leader Kim Il Sung, abandoned by its allies and isolated from the rest of the world, might try to disrupt the games through terrorism. "That's certainly one of their options," said a western diplomat. "I think the odds are high that they would undertake some low-risk activities to dramatize the tension on the peninsula, but I'm not sure about North Korea taking more extreme measures."

The North Korean statement partly blamed the boycott decision on the victory of ruling party candidate Roh Tae Woo in South Korea's presidential election last month. It said Roh's victory leads to "the prolonging of military governments in South Korea," adding that this complicated the task of negotiating with South Korea.