MOSCOW, OCT. 18 -- The Soviet Union's decision to pay all its debts to the United Nations appears to reflect a new Kremlin policy to strengthen the international body as a neutral force in settling regional disputes and to avoid costly Third World entanglements as Moscow embarks on radical economic changes at home.

Western diplomats here said that the Soviet effort to revive the United Nations as a vanguard of international stability could help bolster the organization's flagging morale and credibility. But they suggested that the shift in Moscow's policy -- from previous use of the body as a lectern to blast western foreign policies to current support for an expanded peace-keeping role -- is primarily intended to relieve the burden of Soviet economic and military aid in Third World trouble spots.

The Soviet Union now advocates a more effective U.N. role in such areas as the Persian Gulf. Last spring, Moscow began contributing to the costs of the U.N. peace-keeping force in southern Lebanon at a time when the U.S. Congress had cut American payments in half.

In addition, Soviet leader Mikhail Gorbachev has proposed a sweeping list of new functions for the 159-member body, such as verifying arms control agreements, investigating acts of international terrorism and establishing international standards for human rights.

Since Gorbachev's protege, Eduard Shevardnadze, took over as foreign minister, speeches by Soviet diplomats at the United Nations have been less polemical, The Washington Post's special correspondent at the United Nations, Michael J. Berlin, reported. Moscow is generally regarded now as more amenable to compromises on many issues on the U.N. agenda.

George Sherry, an American who retired this year as the top professional in the U.N. peace-keeping section, said in an interview with Berlin that the Soviets see a greater role for the United Nations as serving the Kremlin's political purposes in resolving costly conflicts while Moscow concentrates its energy and resources on modernizing the economy.

Western reaction to the changing Soviet perspective toward the United Nations has been mixed. Some skeptical diplomats in Moscow said that before taking on additional duties the United Nations should seek to enhance its credibility by redoubling existing peace-keeping efforts.

The United Nations has sponsored indirect talks on the Afghanistan war for four years, for example, but still has failed to bring the Soviet-backed communist government in Kabul and the rebel guerrillas into direct negotiations, let alone to achieve a cease-fire in the eight-year-old conflict.

But other western and Third World diplomats consider the Soviet initiative an important first step in rescuing the United Nations from sagging morale and a history of feckless peace-making efforts.

The Soviets also foresee a broader role for the United Nations in dealing with continuing international disputes in Angola, Ethiopia, Nicaragua and Cambodia.

The Soviet payment last week of $111 million in current bills for both the regular budget and peace-keeping costs was the most striking evidence of Moscow's change of heart toward the international body. In addition, Soviet Deputy Foreign Minister Vladimir Petrovsky said at a news conference in New York that his government would pay within two years the $197 million that it owes in peace-keeping debts dating back 30 years, including operations in Sinai and the Congo.

The Soviet effort to resuscitate the United Nations and its decision to pay its debts to the organization have left the Reagan administration on the defensive.

Congress withheld more than half of its contribution to the United Nations in 1986 for political reasons, and the United States now owes the organization $414 million, including all $212 million for the 1987 regular budget and $50 million due for peace-keeping in the current 14-month billing cycle. U.S. officials expect Congress to appropriate no more than $100 million for the United Nations this year.

So far, the United States has shown no interest in a Soviet proposal last month that the five permanent members of the U.N. Security Council -- Britain, China, France, the Soviet Union and the United States -- revive a U.N. body called the Military Staff Committee, empowered by the U.N. Charter to take joint military actions against aggressors to enforce peace. The three other Security Council members also have shown no interest. In a visit to Moscow last week, however, members of the United Nations Association of the United States, a public interest group, agreed to form a bilateral commission to consider the Soviet proposals and other suggestions, according to Georgi Arbatov, director of the Institute of U.S. and Canadian Studies and head of the Soviet U.N. Association.

In an article published Sept. 17 in the Soviet newspaper Pravda, Gorbachev made an 11-point proposal for strengthening the status and scope of the United Nations. The Soviet leader's initiative called for creating a multilateral center to manage conflicts, including a U.N. hotline to the capitals of the five permanent Security Council members.

He also urged more intensive U.N. efforts to banish international terrorism, alleviate Third World debt and improve world health and environmental problems.

Gorbachev proposed that the United Nations search for new means to make its decisions "more binding politically and morally."

Moscow-based diplomats and western foreign policy experts regard Gorbachev's proposals as a progressive, broad outline for strengthening the body's overall purpose and effectiveness.

But some critics have said the Kremlin so far has seemed reluctant to follow the spirit of the proposals.

While Moscow backs a U.N. resolution for a cease-fire in the Iran-Iraq war, for instance, it is hesitant to approve a U.S. call for an arms embargo against whichever side fails to accept the terms of the resolution -- regarded as the only way to make the cease-fire enforceable.