Alexander M. Haig Jr., in another lengthy day of testimony on his nomination to be secretary of state, declared his opposition yesterday to any further withdrawal of U.S. military forces from South Korea under existing circumstances, and suggested that U.S. power in the Pacific should be increased instead.

Haig's remarks about Korea, one of a wide variety of nations and topics he addressed in nearly five more hours of testimony, appeared to seal the fate of President Carter's controversial effort to withdraw U.S. ground troops from the divided Asian peninsula. Carter's plan, which was among his earliest foreign policy initiatives, ran into strong opposition at home and abroad, and in July 1979 was shelved until after the presidential election.

The third day of questioning of Haig by the Senate Foreign Relations Committee, like a substantial portion of the earlier two, often dealt more with military than diplomatic subjects. The four-star general turned diplomat often spoke of national requirements in terms of armed strength, and seemed most at home on the familiar ground of conventional and nuclear military power.

As the most experienced foreign policy operative in the top rank of the incoming administration of Ronald Reagan, Haig is expected to have an influential voice in a broad range of international decisions, involving the Pentagon as well as the State Department. He has repeatedly expressed the view in his testimony that national power, especially military power, is central to the outcome of diplomatic deliberations.

Among the matters addressed by Haig yesterday, in each case responding to questions, were the use of U.S. military power to protect access to oil resources in the Persian Gulf, the timing of new talks with the Soviet Union on the limitation of strategic nuclear arms, the nuclear vulnerability of the United States in most of the 1980s, and controversial military sales to Taiwan and Saudi Arabia.

Regarding Korea, where about 40,000 U.S. troops are on duty nearly three decades after the armistice between north and south, Haig said the United States has "almost habitually underestimated" the gravity of the North Korean military buildup. At one point he referred to North Korea as a "cruel and insatiable regime."

In this circumstance, it is "vitally important" to U.S. credibility in the world to ensure South Korean security, he said. Referring specifically to ground, air and naval forces, Haig said, "I do not see any justification in the present climate for reductions of any kind in American capabilities."

Referring to talks between the two Koreas, Haig said he was "not aware of any promising prospects." It was unclear whether he had been briefed on the invitation just yesterday by South Korean President Chon Too Hwan to his North Korean counterpart, Kim Il Song, to visit Seoul. The State Department, which had been briefed on the proposal only a short time before it was made, issued a statement calling it "imaginative and constructive."

Haig declined to take a public stand about the case of South Korean dissident leader Kim Dae Jung, who has been sentenced to death by a military court in Seoul. He did say that the case has "certain universal overtones" and could affect Korean relations with Japan. He noted that Reagan has made his position clear -- opposing capital punishment for Kim -- in a message sent to Seoul by his national security aide, Richard V. Allen.

The secretary of state-designate expressed notable concern, on a related matter, about what he called "a growing Soviet threat" both in the western Pacific and, through submarine deployments, in the eastern Pacific near Pearl Harbor. Haig said the United States will find itself at an "increasing disadvantage" if the increased Soviet military presence is seen by others to go unchecked.

Some of the increase in Asian military power, in Haig's view, should come from Japan. But he was cautious in his remarks about Japanese defense policy, suggesting that to berate Japan publicly might contribute to a neutralist swing in that country against U.S. interests.

Haig also showed great caution in his remarks about the Philippines, saying it is important that Washington not "mismanage" its policy in such a way that President Ferdinand Marcos or a successor regime turns unfriendly to the United States and its military bases there.

On other subjects, Haig said:

The United States should attempt to "develop a consensus" with Europe and Japan about protection of oil supplies from the Persian Gulf, "but must be prepared to act, even unilaterally, to secure our access to these vital resources."

He expressed the intention to continue and expand the Carter administration's recent efforts to improve U.S. military strength near the Persian Gulf, but declined to comment on particular sites for U.S. installations.

It will not be necessary to wait for the actual fruition of a "dramatic improvement" in U.S. military power before undertaking negotiations with the Soviet Union on a new arrangement for the limitation of strategic nuclear arms. In his testimony Saturday, Haig spoke of this need, strongly suggesting a major delay in new arms negotiations.

He modified his stand yesterday saying that "decisions" and "allocation of resources" to strengthened U.S. military programs, rather than the implementation of the programs themselves, are his preconditions for new negotiations. Haig said the "womb to boom" this span, as he called it, for major new weapons can be five to eight years.

The "increasing jeopardy" to U.S. strategic nuclear missilery from a Soviet strike must be corrected. Increased U.S. military power, especially as perceived by the Soviet Union, is necessary for an American president to "speak authoritatively" in a moment of crisis, he said.

The United States should follow through on its obligation to provide "purely defensive" armaments to "the people of Taiwan." Haig declined to specify how sophisticated this weaponry would be.While taking no clear stand on the issue of Taiwan relations, Haig has continued to speak of relations with "the people of Taiwan," the existing terminology of unofficial relations.

If he had been in authority at the time, he would have favored the sale of advanced F15 fighter planes to Saudi Arabia.