SEOUL, OCT. 2 -- U.S. pressure on South Korea to open its markets to imports has chiefly benefited Japan, Korean officials said this week.

South Korea lifted import restrictions on 24 items at U.S. insistence during the first half of 1987, but Japan captured 57 percent of the resulting new business, the officials said. U.S. firms' share was only 18 percent, the Ministry of Trade and Industry said in a report.

"The American companies simply did not move fast enough," the report said.

American businessmen and economists here acknowledged that there was some truth in the Korean evaluation, which appeared aimed at blunting further U.S. pressure. But they said the primary reason American companies have trouble selling here remains South Korean import barriers, not Japanese competition or lack of U.S. competitiveness.

"In general, American companies have done as well as can be expected," said James W. Booth, executive vice president of the American Chamber of Commerce in Korea. The South Koreans, Booth added, are "blowing smoke in one direction while reaching around with the other hand and cutting off imports."

South Korea recorded a $7.4 billion trade surplus with the United States last year, and the imbalance is expected to grow this year. In contrast, South Korean trade with Japan was $5.4 billion in the red in 1986.

The trade deficit with Japan, which provides key technologies to South Korea's developing auto and computer industries, is a matter of sensitivity for South Korea, which was a Japanese colony between 1910 and 1945.

At the same time, the trade surplus with the United States has caused frictions in the alliance. As Washington presses for lower tariffs, revaluation of the Korean currency against the dollar and other measures to encourage imports, Seoul complains that the United States does not adequately consider its delicate political and national security situation.

The 24 commodities that South Korea agreed to import more liberally this year increased imports by only $255.4 million, which Americans here say proves their point. South Koreans are so slow to remove import barriers, they argue, that even if American firms captured the entire new market it would hardly affect the bilateral trade imbalance.

But the South Koreans respond that further liberalization does little good if Japan simply increases its lead. A similar report last year said Japan had captured 59 percent of newly liberalized markets, compared with 17 percent for U.S. firms.

"There is such a problem," one U.S. Embassy official said. "A lot of the Japanese companies are next door and very ready to take advantage, while some American companies are less aggressive and export-oriented than we would hope.

"But there are also many American companies that have been very aggressive, have beaten their heads against the wall, and haven't been able to get over the barriers," the official continued.

"We are interested in seeing the market open in general, and we think in the long run that benefits everyone, including the Koreans," the official said.

Products in which Japan seized a quick advantage this year included machine tools, bearings, bulldozers, cameras and kitchen utensils, the trade ministry said.

U.S. businessmen and officials here said they have not had time to study the report.

In the past, they said, the Koreans have sometimes cited relaxed restrictions on commodities for which Washington never sought liberalization, while ignoring requests important to U.S. industry and farmers.