A U.S. Senate delegation said today China has offered to allow Taiwan to maintain its own armed forces after reunification with the mainland, a major concession in Peking's campaign to win closer American ties.

Vice Premier Teng Hsiao-ping told the visiting senators today that Taiwan could assume an autonomous status similar to that of Hong Kong and Macao, one senator said. Teng coupled the effort to calm fears of war in the Taiwan Strait with an unusual endorsement of an increased U.S. naval presence in Asia, a sign of the rapidly escalating Chinese concern with Soviet advances in Southeast Asia and the Pacific, such as the Vietnamese takeover in Cambodia.

Chinese officials here also endorsed, apparently for the first time to an official U.S. delegation, the rearming of Japan, something it has approved in private talks with Japanese officials. Delegation members said they detected great Chinese interest in indirect military cooperation among China, Japan and the United States. One Chinese army leader, they added, said he would welcome a visit by a U.S. naval vessel to a Chinese port.

Sen. Sam Nunn (D-Ga.), the delegation leader who expressed U.S. concern over the security of Taiwan to the Chinese, said he found "most important" Teng's assurance that "Taiwan would not have to disarm" after any formal reunification with the mainland.

The concession seemed to diminish even further Peking's soft-spoken objections to continued U.S. sale of arms to Taiwan after the U.S.-Taiwan mutual security pact expires in a year.

A statement issued by the fourmember delegation on the conversation with Teng said: "It was indicated that Taiwan would retain full autonomy with China for as long as the people of Taiwan would so desire, and in the future Taiwan authorities would possess the same powers they now enjoy.

"Security forces on Taiwan could remain. There would be no requirement that Taiwan disarm in order to achieve reunification. It was indicated that China would not use force to change the system and way of life on Taiwan."

The statement outlined Peking's attitude on regional security by saying: "In discussing the growing Soviet military strength in the Far East, the possibility of defense cooperation by Asian nations, an expanded U.S. naval presence in the western Pacific and a strengthening of Japanese selfdefense forces were regarded favorably by the Chinese."

For what are apparently political reasons that do not actually suit its concept of a Soviet strategic threat to the area, China continued to insist on U.S. withdrawal from South Korea, the senators said. "But outside Korea," said Sen. John Glenn (D-Ohio), "I would say they gave an impression that they would like a continuing U.S. presence."

"Or even an expanded one," added Sen. Gary Hart (D-Colo.).

The Chinese did not entirely rule out the use of force, saying "such a renunciation would reduce the prospects for Taiwan entering into serious negotiations." The statement said force might be used in case of an "indefinite" Taiwan refusal -- defined as lasting several decades -- to talk. Another hypothetical case cited was of Soviet involvement in Taiwan affairs, something the Chinese said they considered highly unlikely.

Glenn said Peking offered as a reward for being able to raise its flag over Taiwan the promise that Taiwan "could set its own standards, even have their own economic system even if it was not communist,... like Macao and Hong Kong," cities on the Chinese coast run by Portugal and Britain with tacit Chinese approval.

Teng, a canny politician who has recovered from an unprecedented two political purges, seemed to be actively lobbying the Senate group for favorable action on such measures as confirmation of an ambassador to Peking and most-favored-nation trading status for China.

As the delegation, which also included Sen. William Cohen (R-Maine), sat down with Teng this morning, he turned to the current chief of the U.S. liaison office, Leonard Woodcock, and said, "I wonder whether Congress will confirm your appointment as ambassador."

Nunn said, "Well, if you took a vote of this delegation it would be unanimous."

"That would be four votes," said Teng, waving the fingers of one hand, "and with mine that's five." Nunn then added that President Carter had not yet nominated Woodcock.

Teng indicated China would take no action on its border with Vietnam unless Hanoi provoked an attack. He said Peking would continue to support the displaced Cambodian government of Pol Pot, now apparently retreating into the countryside.

On trade matters, a high Chinese official disclosed he had recommended that China join the International Monetary Fund. Cohen said he told the Chinese they might have a problem winning most-favored-nation trade benefits from Washington if they did not show a good record in human rights. "They reiterated that this is an internal matter and not a matter to be taken up beyond their own borders," Cohen said.

Nearly every American in Peking, from Woodcock to Sidney Rittenberg, the pro-Peking American expert who served 16 years in Chinese prisons for alleged political crimes, attended a gala banquet in the Great Hall of the People tonight to celebrate the Jan. 1 normalization of relations. The Chinese guest list was headed by Teng Ying-chao, a Politburo member and widow of premier Chou En-lai.

Woodcock, in his toast, underscored the interest in military security that Americans share with the Chinese, saying the U.S.-China relationship "is not only of strategic importance, but central to the foreign policy of the United States."