BRUSSELS -- NATO planners have concluded that the proposed U.S.-Soviet treaty to scrap intermediate-range nuclear missiles would weaken the Soviets' overall military capability by at least as much as NATO's and may result in a small gain for the western alliance in the European nuclear balance.

The Soviets would give up more than three times as many nuclear warheads as NATO in the bargain, an advantage that one top-ranking NATO expert said was "not something to sneeze at."

The Soviets consequently would suffer a relatively greater loss in the ability to strike enemy airfields, command and control centers and rear-echelon positions where troop reinforcements would gather in any war between the blocs.

"Clearly, we don't lose badly. In some ways, we think we have a gain. They give up more targets than we do," another expert at NATO headquarters here said.

While some NATO military commanders are unhappy about losing 108 U.S. Pershing II missiles under the treaty, the United States would still retain the ability to hit Soviet territory from Europe with submarine-launched missiles and bombs carried by F111 aircraft based in Britain.

Eliminating the Pershing II missile force based in West Germany was widely considered Moscow's chief military goal in the negotiation on medium and shorter-range nuclear arms. The Pershing II, with a range of 1,100 miles, is regarded as the alliance's fastest and most accurate weapon capable of striking Soviet soil.

The Intermediate Nuclear Forces (INF) treaty would eliminate only ground-based missiles with ranges of between 300 and 3,500 miles, which are estimated to represent less than 4 percent of the superpowers' total nuclear arsenals.

"We will have a little less accuracy and flexibility," a NATO military expert said. But he added, "We still have weapons systems that cover those {INF} ranges."

The planned elimination of INF makes it more important than before to fulfill current plans to upgrade NATO's battlefield-range missiles and airborne nuclear weapons systems during the 1990s, according to civilian and military experts at NATO headquarters.

The major potential liabilities for the western alliance in the proposed treaty are political rather than military, according to officials here and at NATO's military headquarters in nearby Mons.

"In purely military terms, there is no question that we gain. In political terms, it's different," said Martin McCusker, director of the North Atlantic Assembly's military committee. The assembly is made up of 200 legislators from NATO member states.

One political question raised by the accord is whether it represents a downgrading of the U.S. commitment to defend Europe. The American INF missiles in Europe, which would be dismantled under the treaty, were deployed beginning in 1983 to counter the growing Soviet arsenal of SS20 missiles and to reassure European allies of U.S. determination to help defend them.

The other major concern is that the pact could create political momentum for additional nuclear disarmament accords that might eventually leave NATO vulnerable to the Warsaw Pact's perceived advantage in conventional forces.

Yet there is sharp debate within the alliance over whether Warsaw Pact conventional forces are really superior to those of the western alliance. The International Institute of Strategic Studies in London judges the conventional forces of both blocs to be in rough parity.

NATO experts expressed special satisfaction that the INF treaty would reduce the threat posed to NATO's 35 operational airfields and to ports such as Antwerp, Belgium, and Rotterdam where U.S. troop reinforcements would arrive in case of war. But the Soviets, like the Americans, would retain the ability to hit those targets with aircraft, submarine-based missiles, ICBMS and, in some cases, battlefield-range missiles.

"Both sides will have to make some adjustments. Both have a variety of options left," said Lawrence Freedman, head of the department of war studies at Kings College in London.

Under the planned INF deal, each side is to scrap all of its land-based missiles with ranges shorter than those of intercontinental ballistic missiles, or ICBMS, and longer than those of battlefield-range or tactical missiles. In terms of nuclear warheads, the western alliance will lose a total of about 480 currently deployed warheads. The Soviets, by contrast, would destroy more than 1,500 warheads.

Perhaps the biggest problem is that the treaty provides for scrapping the missiles considered ideal for making a "demonstration" shot early in a war to seek to persuade the Soviets to call off an invasion of Western Europe by Warsaw Pact conventional forces.

In this scenario, a single missile would be fired at a military target on Soviet territory to show Moscow that NATO was willing to use nuclear weapons to defend Europe. The Pershing IIs and, to a lesser extent, the ground-launched cruise missiles serve this purpose better than the alternatives that would be available after the treaty, NATO experts and private analysts said.

The INF weapons are land-based and, therefore, somewhat more accurate than submarine-launched or airborne weapons. "I wouldn't say it's a major problem, but there is a difference in accuracy," a NATO military expert said.

In addition, it is considered preferable for submarines to fire all of their missiles rather than a single one, because a submarine reveals its location to the enemy when it fires. With aircraft, there is the risk that the planes will not get through to their target, although NATO officials said they are certain that Soviet air defenses could be penetrated.

The INF weapons to be dismantled also have the advantage of what is called "visibility."

As ground-based systems, they cannot easily be withdrawn from Europe, as submarines or aircraft can. The INF weapons, therefore, are seen as having a particularly good deterrent role: if the Soviets were overrunning West Germany, NATO would have to "use them or lose them."

But political and military leaders throughout NATO express confidence that the United States would be willing to use other types of nuclear weapons, if necessary, to protect Europe.

"I don't know a senior U.S. official, Republican or Democrat, who doesn't know that the security of the United States is totally tied up in the security of Europe," a senior NATO planner said. "The idea that the Americans would let the Soviets take Europe is very much mistaken, and the Soviets know that."

A final advantage of the INF weapons is that NATO planners believe that they could use a single Pershing II or cruise missile, rather than another nuclear weapon, with somewhat less risk of triggering an all-out nuclear war.

As intermediate-range, European-based weapons, the INF missiles would carry the "message" that NATO was interested in bringing a European war to an end rather than in raising the nuclear stakes to the strategic level.

But NATO planners say that nuclear weapons carried by European-based aircraft, or fired from submarines in European waters, would send the same signal. About 400 submarine-based nuclear warheads already are assigned to NATO's top military commander in Europe for just this purpose.