Withdrawal of the last major U.S. ground combat forces from South Korea would "increase considerably" the risk of conflict with the Communist from the north, the senior American commander has warned.

The 14,000 Gls of the 2D U.S. Infantry Div. based in a likely North Korean invasion route are "very important in the deterrence of war," said Gen. John W. Vessey Jr., commander-in-chief of United Nations and American forces in Korea. "It's the one clear signal to (North Korean President) Kim II Sung that if he starts a war, he fights both the U.S. and R.O.K. armies."

In the first interview since the 54-year-old four-star general took over the Korea commands in October last year from Maj. Gen. Richard G. Stillwell, Vessey this week added his voice as the senior operational commander in Korea to those stressing caution on President-elect Jimmy Carter's troop withdrawal deliberations.

"You 'can say that air and naval forces deter also and that's true," Vessey continued. "But what Kim II Sung might look at is that we had air and naval forces very close to Vietnam and we did not use them. And Vietnam went down the tube."

Vessey said he would be "very reluctant" to see the 2D Div. pulled out and predicted that it would not happen. He said he felt was an intelligent man who had many very good advisers and he noted that Carter promised to consider the Korean situation carefully. "I don't think the division will go," Vessey said.

During the presidential campaign, Carter pledged to withdraw nuclear weapons from South Korea and to phase out ground troops gradually. after Japan and South Korea registered concern, Carter promised to consult both countries and appeared to modify his views.

In a recent statement, Carter said the troop reduction would be very slow, careful and methodical.

Of the 39,000 U.S. military forces in South Korea, the 2D Div. with its 14,000 infantry soldiers is the largest unit-based here under the 1954 mutual defense treaty. Other important army elements are an air defense brigade and a 1,500 man missile command with a tactical nuclear potential. another 8,300 serve in U.S. Air Force units that would give Korea fire support in the event of a war. Carter has declared the air force units would stay.

Vessey emphasized that he was in no position to bargain with the U.S. government over the troop levels, but he made his personal convictions plain. "As the operational commander I tell my bosses what we need to do the jobs they have given me to do," he said. "And we don't have any fat here, we're awfully lean."

Without giving figures, Vessey indicated a slow reduction of non-2D Div. troops was possible as the South Korean forces were able to take over their functions.

North and South Korea are roughly equal in military strength, Vessey said. In a war fought without either side receiving much support from the major powers - China, the Soviet Union and the United States - he said he would gamble on a southern victory.

The 2d Div. is held in reserve. its base at Camp Casey lies between the Demilitarized Zone and Seoul - "in the area through which Kim II Sung must pass to be successful," vessey noted.

"It would be very difficult for Kim to start an attack without becoming involved with U.S. forces," he said. "We would be involved by his choice."

A standard U.S. army briefing refers to North Korea as "a tought treacherous and dedicated enemy. Vessey said the North's "rapid and dramatic" build-up of offensive military strength was matched against "a reasonably adequate defense capability in the south." The South Koreans have a 560,000 strong army.

The American umbrella has protected South Korea's economic growth while maintaining peace and stability in Northeastern Asia, Vessey contended. Kim II Sung knows he cannot unify the Korean peninsula by force under existing conditions, and the chances of maintaining stability are "very high . . . provided force levels and the perception of each side that the capability to carry out its mission remains," Vessey said.

After an American ground force withdrawal, peace or war would depend on !the North Korean president's estimate of the balance of forces and the willingness of the United States to come to the aid of South Korea, Vessey said.

Vessey's strong advocacy of maintaining the key U.S. ground force unit in Korea will be welcomed by Americans here who support the Seoul government and are apprehensive about Washington attitudes. They maintian that press reporting of the birgery scandal and Seoul's repression of human rights has unfairly smeared the success story of a protege nation that has shown dynamic growth and exhibited its loyalty to the United States by sending two divisions to fight in Vietnam.

Vessey said that he did not know what "self-dependence" meant in the context of South Korea - a country of 35 million people facing the nation dedicated to its destruction that has two powerful allies less than 300 miles away, while the only other country committed to help in South Korean defense was 8,000n miles.

The Soviet Union and the People's Republic of China were apparently not willing to support Kim in another military adventure, Vessey said, but it was less clear whether they could avoid involvement if Kim started trouble.

Asked if the United States could be dragged into a preemptive war by South Korea through a staged incident, Vessey replied: "You can come up with all kinds of wild plans, but I can't think of any that would have a chanceof success."