The United States and the Soviet Union have begun discussing formal neutrality for Afghanistan based on the Austrian model as international negotiations on a Soviet pullout enter a potentially decisive phase, according to U.S. and Soviet officials.

The discussions, which were described as preliminary, envision arrangements similar to those that have assured the neutrality of Austria since the withdrawal of Soviet and Western forces in 1955.

Following the signing of the Austrian State Treaty by the United States, Soviet Union, Britain and France, Austria declared perpetual neutrality "of her own free will" and pledged not to join military alliances or accept foreign military bases. The Austrian declaration had been approved in advance by the great powers, and was part of the price of the Soviet agreement to withdraw. Since that time, Austria has become a thriving, Western-style democracy, but with no military ties to NATO.

U.S. policy has long backed a nonaligned, sovereign and independent Afghanistan following the withdrawal of Soviet troops. Neutrality goes somewhat further both as a legal concept and a practical obligation not to cooperate in military activities of other nations.

The Austrian model has been under discussion because it is one of a few instances in which the Soviet Union has withdrawn its military forces and domination since World War II. Two other cases were the northern part of Iran in 1946 and, in 1955, a naval base in Finland.

Soviet officials indicated that Moscow is interested in the Austrian precedent. Any such arrangement would be in addition to the peace accords being negotiated in Geneva under United Nations auspices. Details of a neutrality arrangement have not been worked out, and a senior State Department official said Afghan neutrality also would hinge on decisions by Afghan leaders in power at the time of the withdrawal.

Afghanistan's neutrality is one of a number of unresolved issues that need to be pinned down as a possible "end game" approaches in the lengthy negotiations over Afghanistan, according to administration officials.

The focus of official attention at the moment is the mission of U.N. negotiator Diego Cordovez, who is on a visit to Pakistan and Afghanistan aimed at working out details of the next round of the five-year-old Geneva talks. The Soviets have said they anticipate that the next round will be the last before a withdrawal begins, and there have been many reports that the talks will take place in February.

No specific dates have been proposed by either side so far, according to State Department sources, and there is considerable skepticism among many officials that the talks can be convened so quickly. Both U.N. and U.S. officials said it is essential that the next round of talks succeed, and that success is much more important than timing.

Senior officials in the State Department expressed hope that the Soviets will soon inform Pakistan, and perhaps the United States, of the details of a projected withdrawal. A long-promised visit to Islamabad by Soviet Deputy Foreign Minister Yuli Vorontsov, which is not yet scheduled, is considered a likely means of conveying Soviet decisions.

The basic question in Washington now, as it has been for many months, is whether the Soviet Union will establish credible terms for a military withdrawal and actually go through with it. Some in the administration are highly skeptical, considering most of the Soviet talk about withdrawal to be a sham aimed at neutralizing the issue and ending U.S. and Pakistani support for the Afghan resistance. Others see a variety of signs that the Soviets have no viable alternatives and say that increasingly strong Soviet declarations suggest that a withdrawal is coming.

One of the most politically contentious issues here and in Moscow is the U.S. cutoff of aid to the Afghan resistance. One of four "instruments" previously negotiated in the Geneva talks calls for all outside "interference" to end within 60 days after peace accords come into force.

The United States agreed in December 1985 to act as a "guarantor" of the pact, along with the Soviet Union. The United Nations was told at that time that Washington would play such a role "once the elements of an acceptable agreement had been negotiated," said a State Department official.

This language was cited to make the point that any U.S. guarantee or backing for the Afghanistan accords is conditional on Washington's acceptance of the details of those agreements. And, as of today, the details of a Soviet withdrawal -- including its starting and ending date and a schedule for the pullout of units -- have not been established.

Also yet to be pinned down is whether the Soviet Union would be permitted to continue providing arms and advisers to a post-withdrawal Afghan government, and the degree to which the superpowers would aid the reconstruction of Afghanistan.

President Reagan told the United Nations in October 1985 that the United States "would respond generously" if regional conflicts such as Afghanistan could be resolved through national reconciliation. No details of such a U.S. aid plan have been worked out, officials said. The Soviet Union has recently launched an elaborate program of direct aid for the various Afghan towns and provinces, bypassing the Kabul government.

The timing of a cutoff of U.S. military aid to the resistance has also stirred controversy. Secretary of State George P. Shultz said Jan. 7 that the United States must be convinced that there is "no turning back" on a Soviet withdrawal before cutting off the aid, suggesting that such an action might come later than 60 days after the accords enter into force.

Another question mark is the nature of the interim government that would rule Afghanistan during and immediately after the withdrawal.

Several resistance leaders have publicly rejected Pakistani government suggestions that the Afghan communists be permitted a minority role in an interim regime. The Soviets are demanding a role for the communists, but it is unclear how much of a role Moscow will insist on.

The United States has backed setting up an interim government not dominated by the communists. Recently officials have seemed more hesitant to express U.S. views on this point, saying the Afghan political future is up to the Afghans themselves.