Two weeks ago there were reports in the press that President Carter's plan to withdraw U.S. ground troops from South Korea had been complicated by a new factor -- a U.S. intelligence estimate of the size of North Korea's army.

The full dimension of that new estimate is this:

The U.S. Army and the Central Intelligence Agency now believe North Korea has the fifth-largest ground army in the world, a startling show of strength by a nation of only 17 million people. The new assessment, now in the final stages of interagency review, credits North Korea with a ground force of 560,000 to 600,000 men, about one-fourth larger than previously reported.

The new information has stiffened the resistance by some members of Congress and elements of the American military and civilian officialdom to the planned withdrawal of U.S. ground troops from South Korea in stages by 1982. The first withdrawal of 3,600 troops took place last fall, and 2,400 additional troops are to be withdrawn this year, leaving about 27,000 ground troops and 7,000 Air Force personnel.

But Carter, Secretary of Defense Harold Brown and several other senior officials at political levels are reported to be leaning to the view that the intelligence changes do not fundamentally alter the situation on the ground in Korea, and therefore do not require major change in the withdrawal program.

Carter proposed the Lorean pullout in the early stages of his presidential campaign and has been its strongest governmental advocate since taking office. He is reported to view the intelligence data as only one of many relevant factors, including the large and growing economic margin South Korea holds over North Korea.

The position of the Joint Chiefs of Staff, which informally discussed the intelligence data with Brown last week, is of crucial importance. The uniformed chiefs acquiesced in the Korean withdrawal program with reluctance two years ago, but only on certain conditions. One was that the military balance in the bitterly divided peninsula not be adversely affected.

An explicit premise of Carter's withdrawal, according to informed officials, was that South Korea be able to contain an invasion from the north without the help of American ground troops, given timely warning and use of U.S. airpower.

A sharp increase in the estimate of North Korea's military power compared to that of the South casts that premise into doubt. At a minimum the new data provide the political and policy justification, if one is desired, for taking a new look at a the Carter initiative.

The report that gave rise to the governmental stir originated a year ago with a handful of U.S. Army photo interpreters in the Washington area poring over blown up aerial pictures of North Korea. After laborious study, the analysts reportedly identified about 2,600 North Korean tanks rather than the nearly 2,000 tanks previously credited to Pyongyuang's forces.

These findings set off a major study by the Army to translate the increase in tanks and other materiel into a new estimate of troops. Eventually, 40 analysts were assigned to the review of North Korean military strength. The findings also generated an order from Director of Central Intelligence Stansfield Turner last May that the CIA and the Defense Intelligence Agency make independent studies of North Korean military capabilities as a check on the Army's findings.

The final consolidated estimate is not yet finished. but officials agree that it will show the North Koreans to be "substantially stronger" on the ground than previously estimated.

Army and CIA results, which are in later stages of completion than the DIA estimate, credit North Korea with close to 40 divisions (including separate brigades), nearly twice as many as Pyongyang was believed to have at the beginning of this decade and a large increase from the existing estimate of 29 division equivalents.

Regarding manpower, Army and CIA estimates that North Korea's ground force totals 560,000 to 600,000 men represent a sharp increase from the previous estimate of 440,000 men. Some officials believe the figure may rise further in coming months due to a planned recalculation of North Korean rear echelons and others which so far have not been recounted.

If the new data are officially accepted, North Korea will be credited for the first time in recent decades with an army larger than that of South Korea (560,000), a state of twice the population. The new figures give Pyongyang an army larger than any in the world except for China, the Soviett Union, India and the United States (in that order), all nations with land areas and populations many times that of North Korea.

Most of the new units are reported to have been identified along the coast and near Pyongyang in the central part of the country rather than close to the demilitarized zone (DMZ) that divides North and South Korea. This deployment does not suggest that offensive action is likely in the near future.

Some officials argue that the large reserves, however, make more plausible "worst case" estimates that North Korea could launch a major offensive and still retain enough force to defend against a South Korean or U.S. counterattack.

The South Korean government, which was briefed on the new findings recently, is reported to be taking a "we told you so" attitude. Seoul officials have long claimed that North Korean forces were larger than estimated by the United States. Washington officials said, however, that the new intelligence estimates of North Korean strength are somewhat higher even than Seoul's previous official projections.

Gen. John W. Vessey Jr., chief of the U.S. command in Seoul, reportedly expressed strong conviction during a Washington visit last October that Carter must find a way to set aside the pullout program. Army Chief of Staff Gen. Bernard W. Rogers, according to military sources, is a strong advocate of a go-slow policy on the troop withdrawal.

Sen. Sam Nunn (D-Ga.), an influential member of the Armed Services Committee, reportedly told American officials during a current visit to Asia that he is very disturbed by the intelligence findings, and that he is planning to take a hard new look at the troop withdrawal program early in the first session of the new Congress. Rep. Samuel Stratton (D-N.Y.), an opponent of the withdrawal, has announced an investigation by the House Armed Service subcomnittee he chairs.

Administration officials have emphasized that the new estimates reflect the results of a steady North Korean military buildup since 1970-71 rather than a sudden increase. The United States had been reporting a major buildup all along, but the analysts now say that the increases were much larger than they had realized.

One of the questions being debated in the administration is why the North Koreans have mounted such a large, persistent and costly military program. "Every political animal says North Korea isn't going to start anythin," commented an official, "but every military animal asks, why are they building this force?"

The early 1970s dating would place the start of the military drive about the time Pyongyang was shaken by the dramatic and unexpected movement toward rapprochement between Washington, its superpower adversary, and Peking, its closest ally.

The Chinese on several occasions are believed to have strongly advised North Korea against taking military action, and the Soviet Union is believed to be also strongly opposed to a North Korean thrust across the DMZ. The Soviets have not provided Pyongyang with new warplanes or other advanced equipment for more than five years, according to U.S. sources, and the Chinese provide a minimum of new gear.

One facet of the North Korean buildup has been the creation of major defense industries to build tanks, artillery, ships, submarines and virtually all other weapons of war except for advanced aircraft and electronics and surface-to-air missiles. The creation of this home-grown military base makes North Korea less dependent on both its senior allies, Peking and Moscow.

Across the DMZ in South Korea, the beginning of this decade saw the start of large-scale military modernization programs with U.S. and indigenous financing. Seoul has announced or carried out modernization plans estimated at nearly $10 billion, and received weapons that are more sophisticated than the North's.

During the 1970s, Seoul also has been building a military industry capable of meeting many of its needs for weapons and munitions. With South Korea's economy far outstripping that of the North, the military industrialization is seen as an ominous development in Pyongyang.

The early 1970s, when the North Korean buildup and the South Korean modernization began, paradoxically was the period of a North-South dialogue when the two Koreas for the first time began talking openly and directly to one another. This reflected the opening of the East-West detente involving their respective great power sponsors. However, the North-South dialogue in Korea bogged down toward the end of 1973.

It is now believed that late in 1971, when the Pyongyang-Seoul talks were beginning, North Korea also began secretly digging tunneis from north to south beneath the DMZ. Although the tunneling later was discovered and three large underground passageways were physically intercepted on the southern side, U.S. officials say that North Korea continues the digging operations.

American and South Korean intelligence estimate that more than 5 tunnels have been started beneath the DMZ. Washington officials are as uncertain about the reason for the continued digging as they are about North Korea's objectives in the large-scale buildup of the ground combat forces.