Gen. George Brown, chairman of the Joint Chiefs of Staff, yesterday supported President Carter's plan to withdraw U.S. ground troops from South Korea, but also testified it may not save any money.

"There is some risk involved but it is of an acceptable degree," Brown told a meeting of two House International Relations subcommittees. If Carter's program for a gradual withdrawal is carried out, Brown said, "war would not result from a rational decision" on the part of North Korea. He added that "we cannot account for irrational acts."

Under questioning by lawmakers, Brown said that the uniformed service chiefs as well as the U.S. Army leadership had taken the position in writing that the infantry division to be removed from Korean should be reassigned to the "general strategic reserve" in the United States rather than demobilized.

In this case the saving would be "not signigicant" and the added cost of extra military aid to South Korea would offset even these, Brown testified. Despite the military view, however, he said there had been no government decision yet on the future of the forces to be withdrawn.

Rep. Lester L. Wolff (D-N.Y.), who chaired yesterday's meeting, said on the basis of Congressional Budget Office data that demobilization of the division would save U.S. taxpayers some $900 million yearly. On the other hand, Wolff said reassigment of the division within the United States would cost an extra $150 million in the next five years, mostly in construction to house the unit.

Brown and Under Secretary of State Philip C. Habib, who visited Asia late last month to brief South Korean and Japanese leaders on the U.S. withdrawal program, discussed the matter with Carter in a White House meeting early yesterday before giving testimony to House and Senate Committees.

Habib said the withdrawal is "a natural proper development in our ongoing security relationship" with South Korea. "In arriving at out determination that the ground forces should be withdrawn, we have carefully weighed the military and international considerations involved," Habib said.

Senate Republican Leader Howard H. Baker Jr. (Tenn.) sharply challenged the plan, charging that "we are withdrawing from Korea on the basis of a campaign promise [by Carter] and not a careful and comprehensive review by the U.S. government."

In a written statement Baker charged that the withdrawal will "create uncertainty throughout free Asia, stimulate regional tensions and risk a major outbreak of hostilities on the Korean peninsula.

In a closed session of the Senate Foreign Relations Committee at which Baker was present, lawmakers decided to ask the Central Intelligence Agency for documents on a new intelligence estimate reportedly showing that North Korea is stronger than was previously believed.

Staff briefings yesterday morning by the CIA are said to have expressed concern over the short warning time would be available in the event of a North Korean attack.

The Foreign Relations Committee plans further inquiries into, the withdrawal program, informed sources said. A substantial Senate floor debate is considered likely, perhaps on efforts by Baker to rally opposition to the withdrawal.

Brown and habib would not make public the numbers of men to be withdrawn, the time schedule or the amount of compensatory aid planned for South Korea, saying these remain to be determined. Nor would they discuss in public the future deployment of U.S. tactical nuclear weapons in Korea.

Brown was asked about Maj. Gen. K. Singlaub, who was removed as chief of staff of U.S. forces in Korea for publicly stating that the troop withdrawal plan will lead to war. The Joint Chiefs of Staff do not agree, Brown said, and added that "I did not find any general officers who expressed those views" during his recent Korea trip.

Habib testified that "we believe that it is not in the interest of either the People's Republic of China or the Soviet Union to encourage or support actions which would raise the risk of war on the Korean peninsula," But he conceded that there was a "presumption" but no evidence that the Communist giants would seek to restrain North Korea.