Japan will accelerate implementaton of its "action plan" to reduce its huge trade surplus with the United States, a high-level delegation from Japan's ruling political party said here yesterday.

The "action plan" ran into sharp criticism when it was announced by Prime Minister Yasohiro Nakasone this summer because its provisions would be phased in over a three-year period, which the Reagan administration and members of Congress said is too long.

But Susumu Nikaido, vice president of the Liberal Democratic Party, who is heading a legislative delegation here, said an "extraordinary session" of the Diet (the Japanese parliament) would be held next week to pass many provisions of the action plan so they could take effect by January.

Nikaido's announcement at a press conference marking the end of the delegation's visit here appeared to be part of a large and expensive Japanese public relations campaign to blunt the anti-Japanese mood in America caused by Japan's trade surplus, which is expected to reach $50 billion this year. That is one-third of the total projected trade deficit.

U.S. officials indicated they were skeptical about how much impact the acceleration of the action plan would have on American sales in Japan, especially since they had not yet seen details of the legislation to be presented to the Diet.

Nikaido said the speeded-up program would cover three-fourths of the 1,853 items on which tariffs would either be eliminated or "reduced drastically" and allow the simplification of Japanese standards and certification, which are widely seen as bars to imports.

"The overwhelming reason my colleagues are here," Nikaido said, "is that the LDP, the ruling party of Japan, is deeply concerned about our trade relations with the United States."

His views echoed statements made earlier this week by Prime Minister Nakasone when he declared to foreign correspondents in Tokyo that Japan "will correct any causes of trade imbalances for which it is responsible" and added that "it pains me to hear Japan labeled as unfair. I am determined to work to stop such criticisms."

Nonetheless, strong frictions remain on trade between the United States and Japan, with U.S. Trade Representative Clayton Yeutter saying this week that the Reagan administration must intensify pressure on the Nakasone government to make sure it keeps its pledge to give American products the same access to its markets that its products have in the United States.

While the administration claims success in one area of negotiation -- over telecommunications -- progress in three others has ranged from halting to nonexistent. Yeutter said he wants a successful conclusion of those talks -- including forcing Japan to end barriers to U.S. wood products, which is a contentious political issue for Nakasone -- by January.