LONDON, DEC. 22 -- Completion of the U.S.-Soviet treaty eliminating intermediate-range nuclear weapons has left NATO facing difficult internal debates over modernization and expansion of its remaining nuclear forces in Europe.

The conflicts center on a four-year-old North Atlantic Treaty Organization plan to update short-range nuclear systems and to add new weaponry in additional categories not covered by the accord on intermediate nuclear forces (INF).

Only part of the plan has been implemented, and disagreements over whether, and when, to move ahead with it were set aside last spring in the interests of alliance unity over INF. But Britain made clear at that time that full implementation of the modernization scheme was a condition of its support for the intermediate-range accord.

At the top of the modernization list, in terms of political sensitivity, are replacement of the aging, U.S.-supplied Lance ground-launched, short-range missile and the installation of new, more powerful warheads on other battlefield, or tactical, weapons.

West Germany, on whose soil most of these weapons are based, is resisting such moves. Sources here said that Britain currently is working on a compromise plan to offer Bonn a reduction of up to 600 in the total number of NATO's battlefield nuclear weapons in exchange for agreement to go ahead with modernization of others.

In Bonn, officials said the compromise plan still had not been presented to West Germany, and they declined direct comment on it. But officials from other NATO governments were not optimistic that it would be accepted.

The British position on modernization, along with that of senior officials in the Pentagon, is that elimination of a whole category of nuclear weaponry in Europe has made improvement of the remainder even more important, as long as the Soviet Union maintains numerically superior conventional forces.

British and U.S. officials said they would be pushing for specific decisions on modernization at the April meeting of NATO defense ministers. But other NATO members, including several who would be asked to deploy some of the updated or new weapons, want the decision put off. They argue that with both Moscow and Washington committed to further nuclear reductions, and with the world in an arms-control mood, now may be the worst time to be talking about such nuclear enhancements.

Replacement of Lance and obsolete battlefield warheads is only part of the problem. Also at issue are the proposed assignment to the NATO command of unspecified numbers of U.S. sea-launched cruise missiles and additional European deployments of nuclear-capable U.S. aircraft.

The INF treaty signed early this month by President Reagan and Soviet leader Mikhail Gorbachev covers only ground-based weapons with ranges between 300 and 3,500 miles. All the changes under discussion concern weapons with shorter ranges, or with sea- or air-based delivery systems.

Modernization was agreed upon at a NATO meeting in Montebello, Quebec, in October 1983, when the United States and the Soviet Union were barely on speaking terms.

"What I think you're going to find increasingly emerging," said a senior official in one of the smaller NATO governments opposed to the plan, "is countries claiming that at the point the Montebello decisions were made, we were very far away from the zero agreement on INF now reached."

In the wake of the INF accord, he said, "we should look at the Montebello decision and see whether it is still a rational and desirable way" of pursuing "increased stability in Europe."

The U.S.-Soviet commitment to further negotiations to reduce strategic, or intercontinental, arsenals by 50 percent also foresees limitations on sea-launched missiles that are proposed for European deployment, he said.

"A lot of us will be very concerned lest NATO does anything that makes it more difficult to reach agreement on the 50 percent."

The Soviets already have objected to NATO taking unspecified "compensatory measures" in response to the INF treaty that would undercut the letter and spirit of the new agreement.

A communique released after a meeting of the NATO Nuclear Planning Group of defense ministers in Monterey, Calif. last month reconfirmed the intention to move ahead with the Montebello agreements. "The aim was to reach decisions by spring 1988," a U.S. official said.

But with the next meeting to be held in Copenhagen in April, there is little indication that consensus is near on modernization.

Senior officials just below ministerial rank currently are studying the problems and shuttling among NATO capitals looking for compromises. A number of NATO governments reportedly fear that a confrontation over short-range modernization would unfavorably influence the U.S. Senate's consideration of the INF treaty.

Many of the difficulties concerned with modernization involve SNF -- short-range nuclear forces. In the wake of the INF treaty SNF is the hottest acronym in NATO. The category includes the Lance, NATO's only short-range ballistic missile, and a collection of such battlefield nuclear arms as artillery shells, bombs and antisubmarine depth charges.

In 1979, NATO decided to reduce by 1,000 the 7,000 warheads in this category based in Western Europe, some of which were decades old. At Montebello, a further reduction of 1,400 warheads was agreed upon, a commitment that far overshadowed the commitment to modernize some of the remaining 4,600 weapons and warheads.

Leading the modernization list were the 88 Lance missiles. Most of these are based in West Germany, where 36 are controlled by U.S. forces, 26 by West Germans, and 14 by British, according to figures provided by the International Institute for Strategic Studies here.

During the INF debate earlier this year, some Pentagon officials urged that the 75-mile range Lance should be replaced by as many as 400 modern missiles, with a range of up to 300 miles, to offset an estimated 1,300 comparable ballistic SNF on the Warsaw Pact side.

This was rejected immediately by Bonn, which began to have second thoughts even about the Montebello commitment to replace the existing number of Lances with a longer-range version.

With the removal of the intermediate-range cruise and Pershing II missiles from Europe, the INF treaty had eliminated all ground-launched missiles in Europe except those, like Lance, whose short range ensured that they would likely reach only German territory.

Bonn fears that deployment of new Lance missiles would spark a repetition of the widespread antinuclear demonstrations that accompanied cruise and Pershing deployments in the early 1980s. The West German governmental position, said a disarmament official there, is that there should be a new set of negotiations with the Soviet Union "for the reduction of short-range arsenals to equal ceilings on both sides before any commitment is made to modernize."

But Britain and others argue that it is precisely the imbalance of nuclear forces in the Central European theater, exacerbated by the removal of cruises and Pershings, that has made replacement of Lances a priority.

Officials here acknowledge that a longer range replacement has not even been developed yet. But, said one, the 1960s-era system "has to be replaced early in the mid-1990s, and that means we need to be thinking fairly soon about what replaces them."

There is additional fear that the Soviets might dangle a new proposal before the West Germans, calling for elimination of all short-range systems. Even though NATO is on record as calling for strategic, conventional and chemical weapons reductions before there are any other nuclear negotiations, some governments would find it politically difficult to oppose such a measure.

Bonn also is opposed to more powerful replacements for the nuclear-tipped shells designed for a reported 2,000 nuclear-capable NATO artillery pieces -- most of them, again, based in West Germany. These replacement shells for 155-mm and eight-inch howitzers were part of the Montebello agreement, although their deployment has been delayed by funding problems in the U.S. Congress, as well as by European indecision.

The Warsaw Pact, according to the International Institute for Strategic Studies, has only 1,300 comparable artillery pieces within firing range of the West, although thousands more are based in the western Soviet republics, positioned to move quickly into Europe.

As for NATO deployment of U.S. air- and sea-launched weapons, an increase in the number of NATO-assigned aircraft capable of delivering nuclear bombs is being contemplated. NATO officials last spring urged defense ministers to consider such action, along with new European deployment of U.S. sea-launched cruise missiles, as a means of covering areas of the Soviet Union formerly designated as cruise and Pershing targets.

The proposals do not call for an increase in the West's total number of such air and sea weapons, but rather for transfers from the United States to NATO command.

Britain, which already has an estimated 150 nuclear-capable U.S. F111 bombers based on its soil, has indicated it is willing to take more, and to allow its naval bases to be used to service NATO-assigned U.S. submarines carrying sea-launched cruise missiles.

But other NATO members are not as willing to provide such facilities. The Dutch and the Belgians now see themselves as released from politically unpopular INF deployment decisions, and are not eager to refight domestic antinuclear battles. Norway and Denmark have indicated that an expansion of NATO's nuclear arsenal would not be well received in their region on NATO's northern flank. Both countries exclude basing of nuclear weapons there.

A British official said this country would take more weapons but would like NATO to avoid "the easy option of throwing them all at the Brits."