Japan, worried over mounting allegations from Americans that Japan has benefited from unfair trade practices in semiconductors, said today it would move to settle the issue "quickly and realistically."

The statement followed three hours of discusions between Japanese and American negotiators on the subject in Tokyo today.

Japanese officials declined to disclose the substance of the talks, which appear to have been limited to broad principles.

Before the meetings, the Japanese were reported to be planning to propose setting a "floor price" for semiconductors exported by Japanese companies, so as to reduce allegations of dumping, or illegally selling below cost in foreign markets to capture market share. U.S. trade officials have been less than enthusiastic about "floor prices," however, believing that they might simply bring higher prices to Japanese manufacturers in U.S. markets they have already captured.

Japanese companies sold about $1.6 billion worth of semiconductors in the United States in 1984 and enjoyed a rising market share.

U.S. companies made sales of $680 million in Japan and say their market share has stagnated at around 10 percent, even though U.S. manufacturers dominate sales virtually everywhere else in the world.

Trade in the chips, which are used as building blocks in computers, has long caused tension between the two countries.

It has escalated this year as the U.S. industry has slipped into heavy recession. U.S. manufacturers have filed petitions against the Japanese under Section 301 of the 1974 trade act protesting their tactics in semiconductor trade and asking the United States to take retaliatory action.

One, submitted by the United States' Semiconductor Industry Association, alleges that Japan unfairly discriminates against foreign suppliers in its home market.

The Japanese deny such barriers exist and say U.S. products are simply not as competitive as Japanese ones.

Other complaints, one filed by three U.S. producers -- Intel Corp., National Semiconductor Corp. and Advanced Micro Devices -- and another by Micron Technology Inc., allege dumping in the United States by a variety of Japanese companies.

The Japanese industry has denied the allegations. But on Micron's petition, the U.S. International Trade Commission has made a preliminary ruling against the Japanese.

Fearful of more such complaints, Japanese officials have long considered the idea of establishing a floor price. It would be computed by the Japanese and represent an official estimate of the cost of production. Sales below that price would then be barred.

Both the United States and Japan feel a bit uneasy with the idea, however, because it means new regulation and suggests creation of a cartel and the reining in of free trade. Floor prices have been used in the past, though, with other products, such as steel.

The goal is to create a definition of dumping that both sides agree on. World prices for semiconductors are plummeting. If a producer marks down his prices, notes a foreign ministry official here, "it's very difficult to know whether that's due to economic conditions or intentional dumping."

One Japanese official tonight said no such proposal was made today, while others merely declined to give details of the talks. But Mark E. Foster, Tokyo representative of the United States' Electronic Industries Association, said an official at the Ministry of International Trade and Industry had shown him such a proposal this week and said it would be put to the Americans.

Hiroshi Sugiyama, director general of MITI's Machineries and Information Bureau, told reporters, meanwhile, that the two sides agreed to solve the problem by continuing discussions in three areas -- increased access to the Japanese market, pricing and long-term collaboration.