Japanese Prime Minister Zenko Suzuki warned today that economic aid to South Korea might be restricted if that country's main opposition leader, Kim Dae Jung, is executed.

He said there could be "serious restraints" on assistance programs sought by South Korea and emphasized that this message has been conveyed to the South Korean government.

It amounted to the Japanese government's most forceful representation on the case since Kim was sentenced to death last week by a military court for plotting a rebellion and forming an antistate organization in the early 1970s.

It also reflected the Suzuki government's concern over rising opposition within Japan to the trial and conviction of Kim, who was kidnapped from Tokyo in 1973 in a case that has never been settled to the liking of Japan's left-wing opposition.

Opposition critics contend that Kim did not receive a fair trial and was convicted partly on evidence that violates the spirit of a political settlement reached by the two countries after the kidnaping. They have insisted that Japan apply strong economic pressures and postpone an important ministerial-level meeting with South Korea in an effort to save Kim's life.

The Japanese government has expressed grave concern about Kim's fate and has suggested that relations between the two countries would suffer if the death sentence is carried out. Suzuki's comments today, however, were the first public warnings that the execution of Kim would provoke economic retaliation from Japan.

Government officials said later that no concrete plans had been made for cutting off aid and technical assistance. South Korea looks to Japan for significant amounts of technical assistance and government loans and is hoping for substantial help in pulling its economy out of a deep recession.

This year, the Japanese government has promised to extend yen credits of nearly $100 million to South Korea and has more than 20 experts in that country providing technical assistance on subjects ranging from industrial standards to high-technology products.

Kin was sentenced to death by a military tribunal that found him guilty of plotting insurrection last spring when the country was hit by massive student protests. A second charge accused him of having formed an antistate organization, called Hanmintong, while he was in exile in Japan in the early 1970s.

The political settlement reached after his kidnaping by South Korean agents is said to have stipulated that Kim would not be brought to trial for actions and speeches he made while in Japan. The military court's indictment of Kim appeared to most observers to violate that agreement because it listed several examples of Kim's involvement in Japan with persons active in Hanmintong.

However, the Japanese government apparently has accepted the South Korean contention that those charges are only presented in the indictment as background material, not as offenses bringing the death penalty.

Suzuki's comments today came in an interview with the Japan Broadcasting Co. in a special program nationally televised tonight.

He said that Japan, as a friendly neighbor, hoped for stability and economic progress in South Korea and pointedly referred to existing programs of economic and technical cooperation.

He said that if Kim's case develops in such a way as to cause more worry in Japan there would be "serious restraints" on those economic cooperation projects, even though Japan wishes to continue extending a helping hand to Seoul.

"I would like to clearly state that," the prime minister added.

In answer to another question, he emphasized that the government's position has been clearly explained to representatives of the South Korean government. He also said Japan is appealing to international public opinion in the Kim case.

In the past week South Korean officials have sternly objected to expressions of concern in Kim's case by foreign governments, calling them examples of outside interference in their internal affairs.

Kim's sentence has been confirmed, as expected, by the country's martial-law commander. It can be appealed to a higher military court and to the Supreme Court. If both of them uphold the conviction and death penalty his life could be spared only by intercession of the new president, Chun Doo Hwan.

The issue is likely to become more heated in Japan when the parliament convenes next Monday, with oppositon parties expected to demand a revision of the political settlement reached in 1973.

However, Suzuki said today the government has no present plans for seeking a revision of that settlement.