Defense Secretary Harold Brown warned South Korean President Chun Doo Hwan today that the execution of convicted opposition leader Kim Dae Jung would have a "very significant" effect on U.S.-Korean relations in terms of American and congressional public opinion.

The warning, reported by informed sources, was the most direct American move yet to save Kim.

Kim was sentenced to death in September on charges of fomenting sedition. If the Supreme Court rejects his appeal, only Chun could spare his life by commuting the sentence.Brown's appeal was made on the direct instructions of President Carter, according to the sources.

The appeal drew no assurance from Chun.

"The issue was not resolved," said one source. But shortly after the 1 1/2-hour meeting, the South Korean government for the first time acknowledged through the censored press that the United States is concerned about Kim's fate.

A statement authorized by a South Korean presidential spokesman and quoting "diplomatic sources" said Brown had expressed concern. It quoted Chun as responding that "the case should be entrusted to the fair judgment of the law."

In earlier statements, the State Department had described the charges of sedition against Kim as "far-fetched" and Secretary of State Edmund S. Muskie called his death sentence "extreme."

Since those comments, the Carter administration has engaged in a low-key diplomatic campaign to save the life of the 55-year-old opposition leader who was arrested in the crackdown last May that placed the country under military rule.

Diplomatic sources here and in Tokyo recently have expressed growing pessimism about Kim's fate. They have said that Chun is under strong pressure from key supporters in the military to have Kim executed.

But government rhetoric related to the issue has notably softened in recent days, prompting speculation that there is still indecision on Kim's fate.

By sending Brown, who is regarded here as a friendly figure, the Carter administration was probably playing its best card. Brown is thought to have stressed South Korea's military importance in an administration that frequently criticized the country for violations of human rights and to have helped turn aside initial administration plans to withdraw all American troops from South Korea.

Reliable sources said that Brown did not warn Chun specifically of what the American consequences would be if Kim dies and that he did not promise any specific rewards if Kim is saved.

But his trip in itself was a kind of favor. Since Chun came to power in the military crackdown, the United States had irritated officials here by refusing to hold joint security talks with Soviet Korea.

That ended today when Brown held a long security meeting with Defense Minister Choo Young Bok.