The United States formally asked the U.N. Trusteeship Council today to terminate the agreement by which it oversees the last of 11 territories that were placed under its supervision 40 years ago -- the Trust Territory of Pacific Islands, known as Micronesia.

The Trusteeship Council, whose current active members are Britain, France, the Soviet Union and the United States, operates by majority vote and is expected to adopt a resolution next week approving the U.S. request. But then the issue will come before the Security Council, where U.S. diplomats believe the Soviet Union is likely to block the termination of the trust agreement, with a veto if necessary.

The Soviets have already laid the groundwork by denouncing the draft agreements between Washington and four Micronesian governmental entities as a thinly disguised attempt to "annex" the 2,141 islands, which have just 150,000 inhabitants and are dotted across a vast area of the Pacific Ocean as large as Australia.

During the weeklong debate in the council, Soviet representatives and some petitioners opposed termination of the trust. They questioned the validity of plebiscites that were conducted on the future of the island groups, the levels and guarantees of future U.S. aid, and the prospect that the United States might enlarge its military presence in the islands as an alternative to the naval and air bases now in the Philippines.

Some of the islanders who favor the new status said frankly that the compacts they negotiated with Washington amounted to the marketing of their prime natural resource -- their strategic location -- the way other island groups market their copra.

One of the four political entities, the Northern Marianas, opted for commonwealth status similar to that of Puerto Rico. The other three -- the federated states of Micronesia, the Marshall Islands and Palau -- have each chosen a "compact of free association" with the U.S., which gives them local autonomy but cedes defense arrangements to Washington.

Action in Congress and at the United Nations had been held up by a provision in the Palau constitution banning nuclear weapons from its territory. A new compact agreement was negotiated in January and endorsed by a plebiscite -- which won 72 percent of the vote -- on Feb. 21. Under it, the United States guaranteed that it would not "use, test, store or dispose of nuclear, toxic chemical, gas or biological weapons" in the territory. However, there is a clause permitting the U.S. to bring "nuclear capable" vessels into Palau, and local courts still must determine whether this violates the constitution.

If Moscow blocks the termination of the trust, the U.S. Congress could still act unilaterally to put the four compacts into effect. But the trusteeship council would retain its oversight role over the territory, making U.S. military expansion there somewhat more awkward politically, but no more difficult legally.