The Soviet Union and the Sultanate of Oman announced today that they will establish diplomatic relations, marking a breakthrough in Moscow's long efforts to expand its diplomatic presence in the Persian Gulf.

Among the oil-rich gulf states, only Kuwait has had diplomatic ties with the Soviet Union. Saudi Arabia, the dominant influence in the region, and Moscow have not exchanged diplomats for decades.

The decision to set up diplomatic ties was confirmed at a meeting today in New York between Soviet Foreign Minister Eduard Shevardnadze and Omani Minister of State for Foreign Affairs Youssef Alawi bin Abdullah, following an "exchange of messages" between the two states.

[Oman's decision appears to represent a major setback for U.S. diplomacy in the gulf, Washington Post staff writer David B. Ottaway reported from Washington. The sultanate is second only to Saudi Arabia as America's closest ally in the region.]

The Soviets in the past have been critical of the sultan of Oman, Qaboos bin Said, portraying him as a right-wing puppet of the United States and Great Britain. But at the same time, Moscow has been making overtures to Oman and its neighbors, seeking to widen its contacts in the critical gulf region and the Arabian Peninsula.

Last year, the Soviet Union and North Yemen signed a 20-year treaty of friendship and cooperation during a visit here by North Yemeni President Ali Abdullah Saleh, a Saudi ally. Also last year, Kuwait concluded an arms deal with Moscow, reportedly to cost $300 million.

The Soviets recently have stepped up their diplomatic efforts in the Middle East. They reportedly have probed the possibility of reestablishing ties with Israel and in other ways have shown eagerness to reestablish a diplomatic role in the region.

The Soviet news agency Tass said opening relations with Oman marked "a first important step along the road of developing bilateral relations . . . with a view to strengthening peace and international security."

Western diplomats here were reluctant to describe the diplomatic ties between Oman and Kremlin as a sign that other gulf states -- and Saudi Arabia -- are ready to pursue active relations with Moscow.

They noted, for instance, a recent critical portrait of the Saudi government in Pravda, the Communist Party paper. But they also noted that a son of Saudi King Fahd, here with a Saudi youth soccer team, was given a warm welcome and greeted by Georgi M. Kornienko, first deputy foreign minister.

The strain of the Iran-Iraq war, now in its sixth year, and concern for the stability of the region may have contributed to the Omani decision to open ties with Moscow, diplomats said. Moscow has been the principal supplier of arms to Iraq, while maintaining a neutral stand on the war itself.

A major aim of U. S. diplomacy in the gulf has been to prevent rapprochement between Moscow and the region's Arab oil-producing states. The Omani move could influence other members of the six-nation Arab Gulf Cooperation Council to which Oman, Kuwait and Saudi Arabia belong.

Oman's decision is all the more surprising since the sultanate is the only Arab state in the gulf that has been willing to sign an agreement allowing U.S. military forces to use its airfields and ports. The United States has spent more than $300 million in the past five years to upgrade four facilities there, including an airbase on Masir Island, for use by the U.S. Central Command's rapid deployment force.

The State Department appeared to have been caught off guard by the development. An official said the Omani decision was "unexpected" but refused further comment.

After his meeting with Shevardnadze, Alawi informed the United States of his government's decision in a meeting with Vice President Bush. During the 30-minute meeting, Bush renewed the U.S. commitment to continuing strong relations with Oman, according to a spokesman for the vice president. In addition, Bush expressed appreciation for Omani support of American interests, particularly in the Middle East.