Twenty-nine members of the House asked President Carter yesterday to deny $450 million in pending Export-Import loans to South Korea because of a "repression of human rights" on the part of military leaders there.

Rep. Don Edwards (D-Calif.) and others said in a letter that denial of the loans "will send a clear message . . . without any destabilizing security implications" a time of "eroding democratic freedom" in Korea.

The lawmakers gave prominent mention to the case of opposition political leader Kim Dae Jung, who was formally indicted yesterday on treason charges which carry a potential sentence of death. Kim will be tried by court-martial this month.

State Department spokesman David Passage said the United States will be "watching very closely" the trial of Kim, and reiterated the view that nothing in the charges against the veteran political figure " seemed to us to have much foundation."

Passage also expressed U.S. objections, phrased in tentative and indirect fashion, to the military purge of the Korean press reportedly under way. "As a general proposition, any politically motivated dismissals of journalists would be particularly unfortunate and wholly inconsistent with our own national view of the necessity to protect the freedom of the press." Beyond this, Passage refused to comment on the reported purge.

The case of the jailed opposition politician, who won more than 4 million votes in his unsuccessful presidential race in 1971 against then-president Park Chung Hee, has aroused international attention and expressions of concern from a large number of Asian, western and nonaligned nations.

Carter administration officials said strong concern about Kim has been expressed to Korean military and civilian authorities in recent weeks.

Both U.S. and Korean sources expressed doubt that Lt. Gen. Chon Doo Hwan and the other Korean leaders have made up their minds about the fate of Kim. It is taken for granted that he will be convicted, and several sources anticipated that he will be sentenced to death in the lower court.

The greater question, according to these sources, is whether the sentence then will be reduced by higher courts or commuted by civil authority.

Execution of Kim on treason charges would be likely to create a storm of criticism abroad and in some regions of Korea. Thus, U.S. officials said the politicians's fate may be a crucial test of the Korean regime's sensitivity to U.S. and world opinion as well as of its domestic policies.