South Korea has asked Japan for a huge increase in economic aid based on a new claim that its heavy burden of military spending helps defend Japan. Tokyo's refusal to entertain the request has touched off a nasty feud between the two countries.

During talks in Tokyo two weeks ago, South Korean Foreign Minister Lho Sin Yong asked his Japanese counterpart, Sunao Sonoda, to provide $6 billion in low-interest loans for economic development over a five-year period beginning in 1982.

Sonoda suggested that South Korea could expect a marginal increase in aid over last year's $83 million, but rejected Seoul's bid to tie such aid to its new concept of military burden-sharing.

By linking the issues of aid and military security, however, Seoul has deftly turned the tables on Tokyo's diplomacy. In response to strong pressure from the United States that Japan should boost its current level of defense spending, Japanese officials have argued that Japan prefers to meet its obligations to Asian security by increasing nonmilitary assistance to help ensure political stability in the region.

The South Koreans intend to hold the Japanese to their word. Officials in Seoul argue that South Korea spends roughly 6 percent of its gross national product on defense, while the Japanese, whose gross national product is 20 times larger, spend only 0.9 percent.

South Korea, these officials contend, is providing Japan with a strategic backstop against the potential threats from communist North Korea and, in effect, is underwriting the defense of Japan.

Underlying these assertions is the fact that South Koreans chafe over what they view as an economic relationship that vastly favors Japan. According to government statistics here, South Korea has piled up $20.5 billion in cumulative trade deficits since 1965: last year alone the figure was $2.8 billion. Japan is also the biggest foreign investor in the South Korean economy and its factories and trading offices maintain a highly visible and profitable presence here.

But the Japanese have quickly backed away from any commitment that would, in principle, draw them into backing South Korea's military efforts. The Reagan administration's hawkish stand on military issues, reliable sources here said, has helped prompt Seoul in making its new demands for defense-linked economic aid from Tokyo.

South Korean President Chun Doo Hwan amplified the point when he told a visiting Japanese opposition political leader last week: "Japan should be well aware that a war, should it break out on the Korean Peninsula, could not be contained within the peninsula, threatening to spread to other parts of Asia."

In speaking to fellow Liberal Democrats in Japan later in the week, Japanese Prime Minister Zenko Suzuki suggested that the antiwar provisions of Japan's constitution would prohibit providing economic assistance on the basis of military security considerations.

Japan will consider South Korean requests from a purely economic standpoint, one year at a time, and only after Seoul has presented proposals for specific, nonmilitary development projects, according to a Japanese Foreign Ministry spokesman.

The two sides will thrash out their differences, he said, when top South Korean and Japanese ministers meet for scheduled talks in Seoul on Sept. 10 and 11.

Political relations between the two neighboring nations have always been sticky, especially in the wake of president Park Chung Hee's assassination in October 1979. But now South Korea is insisting on a more equal and open relationship. One of the complaints voiced by the South Koreans is that postwar ties have been handled by an "old boy" network of Japanese businessmen and politicians and their South Korean contacts involving alleged profiteering.

"Now the Koreans are saying that things are going to be different," one high government official in Seoul said. "If the Japanese want to talk to us they are going to have to talk up front" to the South Korean government. These efforts to put relations on a more equal footing have "perplexed" the Japanese, he said, "but they are slowly getting the message."

The recent meeting of foreign ministers Lho and Sonoda in Tokyo was intended to help remove some of the rancor in relations between the two countries. But on the eve of the talks, Sonoda sent South Korean tempers flaring with an offhand remark to Japanese reporters referring to the long, difficult history of two-way ties.

According to reports in the South Korean press, Sonoda admitted that Japan had erred in past policies toward Korea, whose colonial occupation by Japan ended with Japan's defeat in World War II. But, the senior diplomat went on to say, Korea had also tormented Japan in prehistoric times, presumably as the result of an early, unrecorded invasion. A Japanese Foreign Ministry spokesman confirmed that a high official made such remarks but declined to attribute them to Sonoda.

As a result, the outspoken foreign minister churned up a widespread sense of outrage in South Korea where public sensitivities dwell on what are perceived here as Japanese attitudes of superiority and Japan's attempts to dominate the South Korean economy.

Police officials here reported last week that they had strengthened security around the Japanese Embassy after receiving a telephoned threat to blow up the building following Tokyo's rejection of Seoul's aid proposal. Reliable sources in both Seoul and Tokyo said that the standoff over the aid issue is certain to delay the scheduling of a meeting between Chun and Suzuki originally expected later this year. Suzuki said last week that he would not meet with Chun before an agreement on the level of Japan's aid was at hand.

Japan is likely to come up with a compromise offer to cool South Korean tempers, these sources said, but it is almost certain to fall far short of Seoul's expectations.

Asked what South Korea would do if Japan offered $1 billion or $2 billion, a high government official here said jokingly, "We would invade Japan. It's not the money that's important. We want to make the Japanese feel guilty."