West European leaders are warning the Reagan administration that Washington must make an effort to resume nuclear arms talks with the Soviet Union soon or risk a serious erosion of support in Europe for NATO deployment of a new generation of nuclear missiles there.

The warning that a failure to create at least the appearance of a dialogue with the Soviets on arms control could strengthen opposition in Europe to the new missile deployment has been conveyed in Washington by the foreign ministers of Britain, France and Italy and by British Prime Minister Margaret Thatcher during visits this month. It is to be reinforced by West German Foreign Minister Hans-Dietrich Genscher when he visits March 8.

From the European perspective, this warning and the larger concern about keeping communications open with Moscow have had at least as high a priority in these introductory contacts as the Reagan administration's call for support on El Salvador has had for Americans.

British sources say the Europeans also have had a moderating influence on the administration's approach to El Salvador. Calling the present El Salvador government "as unsavory as its opponents," they noted that yesterday Secretary of State Alexander M. Haig Jr. publicly stressed the need for social reform there as well as an end to outside military aid to leftist rebels.

On arms control, the Europeans are asking President Reagan and Haig to recognize the domestic political problems facing the West German, Italian and British governments in going ahead with deployment of the new NATO nuclear weapons on their soil -- and those facing the Belgian and Dutch governments, which have been forced to postpone decisions to join them.

Thus far, the discussion has been framed in general terms. France is not part of NATO's military organization and has been careful in its private and public comments to steer clear of the specifics of the theater nuclear force modernization plan.

But the West German government is expected to suggest specifically that the Reagan administration resume the adjourned Geneva talks on so-called "Eurostrategic" nuclear weapons, those targeted for Europe. As a first step, officials in Bonn have told Washington Post correspondent Bradley Graham, the West Germans want the NATO allies to meet in Brussels this spring to work out a coordinated position to present the Soviets. The U.S.-Soviet talks opened in Geneva last October as a result of a Soviet initiative conveyed through West German Chancellor Helmut Schmidt.

Reagan said after his meeting with Thatcher at the White House Thursday that he joined her in reaffirming the Atlantic Alliance decision in December 1979 to modernize NATO's medium-range theater nuclear force weapons in Western Europe "and to pursue arms control efforts at the same time, in parallel."

This is what the Europeans wanted to hear. But they are worried that the Reagan administration will not move as quickly or purposefully to resume arms talks with Moscow as it will to push for a Western arms buildup.

European diplomats say this could make their governments much more vulnerable to growing left-wing opposition to higher defense spending and deployment of the new NATO nuclear weapons on European soil.

The Europeans say they are getting their message across, but they do not know whether Washington will act on their advice. One official in Bonn said the West German government had detected in its discussions with the Reagan administration "a hang-up about arms control."

Thatcher and the European foreign ministers visiting here also have found the new administration still largely unprepared to make specific policy decisions on this and the other issues crucial to its relationship with the allies.

Thatcher tried to put the best light on this by telling reporters here that she was "very impressed with the amount of care and thought that is being given to working out the policies on almost each and every one of the international problems and not to pronounce before those policies are fully worked out."

Senior European diplomats say they see the Soviet and NATO buildup of theater nuclear weapons in Europe as the central issue between East and West today. Their comments suggest the administration has not been forthcoming on whether it will seek a new version of the strategic arms limitation treaty (SALT II).

Reagan had attacked the original version negotiated with the Soviets by the Carter administration, which declined to present it to the Senate last year after the Soviets sent troops into Afghanistan.

The NATO allies decided unanimously in December 1979 to deploy 108 Pershing II and 464 ground-launched cruise missiles in Western Europe beginning in 1983 to counter the ongoing Soviet deployment of medium-range nuclear missiles targeted on Europe. But the Europeans insisted that the deployment decision be coupled with a determined effort to negotiate an agreement that would limit the Soviet deployment and make some or all of the new NATO nuclear missiles unnecessary.

West Germany also wanted another European ally besides Britian to join it in allowing the New NATO missiles to be deployed at bases on its soil. Only Italy has firmly decided to do so.

Even in Britain, where Thatcher's government has been a leading advocate of the decision to deploy the new NATO missiles and has already designated bases for them, advocates of unilateral disarmament are staging mass ban-the-bomb marches again and have gained considerable influence in the opposition Labor Party, whose new leader, Michael Foot, is a veteran disarmament campaigner.

A similar development is occurring on the left in West Germany, putting uncomfortable pressure on the Schmidt government. Bonn has reaffirmed its commitment to allow the new NATO missiles to be deployed there, but it would like the Reagan administration to help provide political cover by moving soon on arms control in a forum like Geneva while deciding what it will do about SALT.

The Geneva talks were adjourned with both sides disagreeing on just what should be included in the category of "Eurostrategic" nuclear weapons, as opposed to intercontinental weapons the United States and the Soviet Union have targeted on each other.

West German officials suggested that Soviet President Leonid Brezhnev made a shrewd tactical move and put pressure on NATO with his recent offer to negotiate with Reagan. But, like Thatcher in remarks to congressmen and reporters here, officials in Bonn pointed out that his offer of an immediate East-West moratorium on deployment on new theater nuclear weapons in Europe would only preserve current, growing Soviet superiority.

The Soviets have accelerated their deployment of the medium-range missiles, according to the West Germans, and now have 200 of them -- each armed with three independently targeted warheads -- aimed at Western Europe, West German officials say NATO can never catch up with the Soviets unless an agreement is reached limiting the number of weapons on each side. This conflicts with the belief of Reagan advisers that the Soviets can be forced to make more concessions to the West if NATO first beefs up its nuclear arsenal.

The West German case for new arms control talks also was strongly supported by Italian Foreign Minister Emilio Colombo in his talks here this month, according to diplomatic sources. The Italian government believes it should have more influence in these matters because deployment of the new NATO missiles in West Germany now hinges on Rome's commitment to also allow them on Italian soil.

Colombo made clear to the Reagan administration that Italy also expects to be involved in all future big-power talks among the allies. Some U.S. officials see this as threat by the Italians to back down on their missile commitment if they are excluded from high-level decision-making in the alliance.

Italian sources say this commitment is not in doubt, but Colombo did stress that "we have a right and responsibility to be involved in the decision-making process. We are very firm about this."