President Carter, bowing to serious opposition in Congress, yesterday ordered a slowdown in the planned withdrawal of U.S. ground troops from South Korea.

The White House announced that only one combat battalion of 800 troops, rather than a full combat brigade of 3,400 troops, will be withdrawn from Korea this fall. The schedule for a pullout of some noncombat personnel will be unaffected.

Carter did not abandon his commitment to pull out all U.S. ground troops, about 32,000 in all, over the course of four or five years. But the postponement in the face of congressional difficulties raised new questions about the eventual fate of the controversial plan.

In a written statement, Carter described his decision as a "prudent" measure in view of the possibility that Congress might fail to act now on his proposals to compensate Korea for the scheduled withdrawal by supplying $800 million in equipment and $275 million in arms credits.

Congressional leaders have reported that the aid proposals are in jeopardy due to a combination of the Korean bribery investigation and the basic opposition of many lawnakers to the pullout program.

Defeat of the Korea aid program - a distinct possibility if Carter had pushed ahead now - could have damaged the credibility of U.S. support for South Korea. The consequences of such an outcome might have been serious in both North and South Korea as well as elsewhere in Asia.

The Joint Chiefs of Staff, never enthusiastic about withdrawing U.S. ground troops from Korea, had conditioned their acquiescence on enactment of the compensatory aid. For Carter to move ahead with major pullouts without being able to supply the aid would have risked a public showdown with the uniformed military.

Chairman Clement J. Zablocki (D Wis.) of the House International Relations Committee praised Carter's announcement as providing "a reassurance" that he intends to provide the promised aid as the U.S. troops withdraw. Zablocki said the action "takes some of the pressure off" of the need for immediate action on the aid program, but said he expects it can be passed later this year.

Sen. John H. Glenn Jr. (D-Ohio), chairman of the Asian subcommittee of the Senate Foreign Relations Committee, said on the other hand that Carter is 'wrong' even to begin withdrawing troops without supplying the promised aid. Glenn said he fears that the partial pullout, in the present climate, will be taken as sign that "America is pulling back all over the world."

White House officials said that South Korea has been informed of Carter's slowdown and expressed approval.

The officials said there was no connection between the decision and the current South Korean airliner incident, and they said there had been no change in the military situation in Asia that affected Carter's thinking.

According to the original plan some 6,000 U.S. troops were to be withdrawn from South Korea this year in the first increment of the larger pullout.

Carter said yesterday that 2,600 noncombat personnel will be withdrawn on schedule. According to the Pentagon, about 1,400 have already left.

One combat battalion of some 800 men is to be withdrawn in December, the announcement said. However, the planned withdrawal of two other combat battalions, a brigade headquarters and supporting elements totaling 2,600 men is being postponed until next year, Carter said.

Carter said his plan to increase U.S. Air Force strength in Korea from the present 60 F4 Phantom fighters to a new strength of 72 jets is unchanged.

"Peace and stability on the Korean peninsula and in Northeast Asia are vital to our national interest," Carter said in his statement.

A White House adviser, briefing reporters, maintained that the U.S. commitment to South Korea would be unaffected even after the withdrawal of all ground troops because of air and naval forces in the area, the U.S. Korean defense pact and other factors.

The official conceded, in response to questions, that mainland China would like the United States to keep its military forces in Asia, including Korea, as a counterweight to the Soviet Union. But he added that Carter still intends to proceed with the Korean withdrawal over the next few years.