West Germany and the Soviet Union have agreed to consult regularly about nuclear weapons in Europe during the course of forthcoming Soviet-U.S. arms reduction talks in Geneva.

The consultations, suggested by the Kremlin during a visit here by Soviet President Leonid Brezhnev that ended today, are to be what one informed West German source described as "an accompanying dialogue" to Bonn's discussions with the United States.

Up to now, the West German government of Chancellor Helmut Schmidt has been involved in helping to shape the U.S. negotiating position through multilateral consultations within the Atlantic Alliance.

The new Soviet link would appear to enlarge West Germany's role in the negotiations. American officials here, surprised to hear of the development, said it could complicate the U.S.-Soviet bargaining, which begins Monday, by diffusing at least the form in which the West approaches the Soviet Union.

West German officials sought to play down the significance of the move, portraying it as a natural outgrowth of West Germany's ties with the Soviet Union and its particular concern about nuclear weapons.

"We intend to maintain continued contacts with the Soviet Union," Kurt Becker, Bonn's chief government spokesman, told reporters following Brezhnev's departure today. "We are the ones mainly concerned, either as the target of Soviet rockets or the place where U.S. deployment of new medium-range nuclear missiles will take place."

A West German source who was close to the German-Soviet discussions here said the new contacts in no way mean that Bonn wants to move away from the United States. He said West Germany did not want to disturb the Geneva talks, enlarge them or complicate them. The West German government, he said, simply assured the Soviets that "we will stay in contact" during the negotiations.

The arrangement was described here as not constituting any form of German-Soviet back channel, but rather as a working relationship.

A joint eight-page communique issued just before Brezhnev left today reflected the gulf that was sharply defined here between the U.S. and Soviet starting positions in advance of the negotiations. The communique declared that the aim of the talks should be a balance of medium-range nuclear arms "at the lowest possible level."

The communique also dealt with Afghanistan, detente, the Helsinki agreement and the recent multibillion-dollar gas pipeline project recently agreed upon by the two countries. There was no specific reference, however, to the planned West German-Soviet contacts about nuclear weapons talks.

In assessing Brezhnev's four-day working visit here, West German officials today stressed that it gave Schmidt the chance to make clear that his government would follow through on its commitment to accept the new U.S. Pershing II and cruise missiles if the Geneva talks produce no result by the summer of 1983.

According to Becker, Schmidt reminded Brezhnev of the West German leader's threat to resign if the Atlantic Alliance double decision, which couples the planned missile deployment to U.S.-Soviet negotiations on limiting European-based nuclear weapons, was not observed.

While the Soviets showed no changes in their basic position in advance of the Geneva talks, West German officials point to two positive elements in Brezhnev's statements here. One was his willingness to negotiate seriously about substantial cuts in Soviet land-based systems. The other, related to Brezhnev's proposal for a mutual East-West moratorium on deployment of medium-range missile systems in Europe during the Geneva talks, was his willingness to withdraw hundreds of nuclear rockets in advance of a final settlement providing certain terms are met.

No specific arrangements have been made to structure the special Bonn-Moscow contacts, according to members of Schmidt's Social Democratic Party who were informed of the development in a parliamentary group meeting with the chancellor yesterday. Rather than requiring any new formal or institutional tie, the Bonn-Moscow exchanges about nuclear weapons are expected to rely on existing diplomatic channels and, from time to time, other high-level meetings.

"This is to say that just because we are not at the negotiating table, the Soviets won't stop talking with us," said Horst Ehmke, a senior Social Democratic deputy.

Ehmke said that if the Soviets had not suggested such exchanges, "we would have done it anyhow" in view of all that West Germany believes is at stake in the negotiations.

"I don't see much of a problem in it," Ehmke said, noting that other West European governments may also feel compelled to undertake similar contacts with the Soviets. "The main thing is that we all tell the same story to them."

Ehmke argued that the contacts would be of definite benefit to the United States by providing another opening through which the West could underline its steadfastness on a common negotiating position.

Schmidt personally has played an instrumental role already in initiating the alliance missiles decision by focusing attention on Soviet nuclear superiority in Europe in 1977, and within the past two years, the West German leader has portrayed himself as partly responsible for bringing Moscow and Washington to the negotiating table in Geneva.

He has also taken credit for urging President Reagan to adopt the "zero option" as the basis for Washington's opening offer to the Soviets. Under this proposal, the United States would drop plans to deploy its new missiles in West Germany and other West European countries if the Soviet Union scrapped its land-based, medium-range SS20, SS4 and SS5 missiles.

Much of the need for the new Western weapons and the special set of U.S.-Soviet arms talks has come about, in Schmidt's view, because of the way the superpowers conducted the Strategic Arms Limitation Talks (SALT) during the past decade. They excluded from consideration the medium- and short-range nuclear weapons based in Europe.

To associate the European allies more closely in the shaping of U.S. negotiating policy, a special consultative group was set up in the North Atlantic Treaty Organization at the time of the 1979 decision to deploy a new generation of nuclear missiles in Europe. It is this group that Bonn has used to help formulate the U.S. position.

But no other major Western country has as close and continuing relationship also with the Soviet Union as West Germany, due to both historical and geographical factors. While Bonn officials try to avoid claims for any special relationship, it sometimes pushes the West Germans into playing a special role in East-West relations, and this may be happening again with regard to the Geneva negotiations.

Indeed, the current meeting is an example of routine annual exchanges of emissaries established under the 1970 reconciliation treaty between Bonn and Moscow.

As publicly outlined, the opposing U.S.-Soviet nuclear positions are roughly as follows:

The Americans want to approach the talks in stages and limit the first stage to medium-range, land-based nuclear missiles in Europe, bargaining on the basis of the zero-option solution.

The Soviets accept the idea of stages, but insist that America's "forward-based systems" -- meaning nuclear aircraft stationed in and around Europe on land and sea -- plus French and British nuclear systems be included as part of any agreement that would bring reductions in land-based missiles.

The Americans say the Soviets have a substantial superiority in the field of land-based, medium-range nuclear systems. The Soviets do not dispute those numbers, but say there is rough parity if the U.S. forward-based systems and West Europe's nuclear weapons are counted.