The Reagan administration stopped short of declaring victory in its effort to make sure American suppliers can compete in Japan's newly denationalized telecommunications market out of concern that the Japanese government will back off its latest commitments, administration officials said yesterday.

"If we claim victory, there's no motivation for Japan to move forward," one official said.

Other administration officials said, moreover, that Japan has to be watched closely to make sure it implements the new regulations in a way that does not discriminate against foreign companies. "We are going to watch very closely to see that, in practice as well as theory, the system is nondiscriminatory," one official reported.

Also, industry members, congressional trade specialists and some mid-level trade officials remain skeptical about the promises made over the weekend by Prime Minister Yasuhiro Nakasone to special envoys sent to Tokyo by President Reagan.

"We want to hear cash registers ring. That is the only meaningful measure," said Brian Wynne of the American Electronics Association.

The talks over two key issues will continue in Tokyo over the next two weeks as a result of Nakasone's commitment to reduce the number of standards that equipment will have to meet to qualify for sale in Japan.

"This will be done with an aim of making the regulatory process equitable between Japan and the U.S. and will be based on the principle that the choice of terminal equipment and telecommunications protocols should be left to the users," Moriya Koyama, vice minister for posts and Telecommunications, wrote Commerce Undersecretary Lionel H. Olmer on Monday.

Japan originally listed more than 50 standards, but dropped the number to 30 in regulations released Monday, while the United States is pressing for a single standard: harm to the network.

The Koyoma letter to Olmer, who with National Security Council aide Gaston Sigur made the weekend trip to Tokyo at President Reagan's request, reportedly was to be released yesterday as part of an administration effort to blunt protectionist pressures in Congress by providing some documentation to show that the telecommunications talks were successful.

However, as a result of a dispute within the administration on how far it should go in declaring it had achieved its objectives in the trade talks, the exchange of letters between Olmer and Koyoma were kept under wraps, administration officials said. Copies of the letters were made available to The Washington Post.

Administration officials and Senate Republicans, meanwhile, reacted cooly to Japan's decision to send Deputy Foreign Minister Reishi Teshima to Washington to explain its trade policies. His trip is a reaction to Senate and House resolutions urging President Reagan to punish Japan for what is seen as protectionist trade policies and their role in the United States' $36.8 billion trade deficit with that country.

The sources said Teshima would do better listening rather than explaining so he can learn first-hand about the new mood of Washington, especially in Congress.

"If he comes to listen, he'll get a real earful, and it will be useful," a U.S. official said. "He'll be able to tell people in Japan what the real mood in Washington is. Some there see it as just another trade crisis, but we see it as qualitatively different from what we have faced before."

It is unclear who Teshima will be seeing on his short trip here. Although a major part of his mission, as outlined in Tokyo, is to explain Japan's position to Congress, few legislators are expected to be here because the Easter recess starts Friday and Teshimi will not arrive until this afternoon.