Not to be outdone by the French and British officials who visited Washington last month in a spirit of cooperation with the new Reagan administration, West German Foreign Minister Hans-Dietrich Genscher left today for his own round of top-level talks in the United States, but with an even more delicate task.

Seeking to allay U.S. doubts about West Germany's reliability and suspected drift within the Atlantic Alliance, Genscher is expected to underscore Bonn's solidarity with Washington as a cornerstone to Western unity.

At the same time, he will be looking for assurances that U.S. positions on East-West relations and Third World developments, when eventually reformulated by the Reagan team, will remain flexible enough to accommodate special West German interests.

To some degree, Genscher will simply be reinforcing positions of general West European concern: the urgent wish that U.S.-Soviet talks on arms limitation resume; the plea for a political rather than a military solution to the civil strife in El Salvador, and Western Europe's desire to pursue its own initiative in the Middle East exploring possible solutions to the Arab-Israeli conflict.

He is expected to put particular emphasis, however, on two aims that are of central importance to West German foreign policy -- furthering cooperative relations with the Soviet Union and stabilizing conflict zones in the Third World -- aims that could be upset by the Reagan administration's tougher line against the Soviet Union and by any tendency to distinguish sharply between U.S. friends and enemies within and among developing countries.

The chances for coherent and effective Atlantic Alliance policies to form under President Reagan's leadership depend largely on how successfully Bonn and Washington can compromise differing views on the timing of arms control talks, on detente and on the conduct of North-South relations. Because of the difficulties involved, Genscher has been granted considerable time with Secretary of State Alexander M. Haig Jr. Genscher also will meet with Reagan, Vice President Bush, Defense Secretary Casper Weinberger and members of Congress.

Genscher will want to appear, at least publicly, in harmony with the new administration's designs. The warming of relations with London and Paris -- following recent visits by British Prime Minister Margaret Thatcher and French Foreign Minister Jean Francois-Poncet -- prompted political commentaries in the West German press worrying that Bonn would be left on the sidelines while the other major Western powers closed ranks with Washington.

Genscher must reassure Washington about the political strength of the Bonn government to make and deliver on promises, Strains in the coalition between Genscher's Free Democratic Party and its senior partner, the Social Democratic Party, have eroded confidence abroad in West German stability.

Chancellor Helmut Schmidt, under an unusually vigorous political attack by the left wing of his Social Democrats, is scheduled to visit Washington May 20.

During Genscher's visit, the West Germans expect to engage in a stock-taking on East-West issues with U.S. officials. The West Germmans will stress their special relationship with the East Bloc, codified during the past decade by a series of cooperative treaties that Bonn intends to honor.

They will also point one side of the Atlantic to the other: The disillusionment and skepticism expressed in the United States about cooperative relations with the expectations here of further positive East-West developments.

In light of the willingness to negotiate on arms voiced by Soviet President Leonid Brezhnev at last month's Soviet party congress, Genscher is expected to seek a shortening of the pause on the U.S. side. He probably will argue that a resumption of arms talks -- particularly those on so-called Eurostrategic weapons that started in Geneva last October -- are necessary in order not to risk erosion of support in Western Europe for deployment of U.S.-built, medium-range nuclear missiles starting in 1983.

West Germany's immediate objective on this point is to promote an early meeting of the Special Consultative Group in the North Atlantic Treaty Organization, so experts can work out a negotiating position in time for approval by NATO ministers at a meeting in Rome early in May.In addition, Genscher hopes for a quick Western reply to the Soviet offer to extend the area for European confidence building measure, so fina' action can be taken at the Madrid Security Review Conference on France's proposal, endorsed by Washington, for a European disarmament conference.

Genscher is expected to explain that West German left-wing political opposition is thwarting the proposed sale to Saudi Arabia of West German military equipment that would be helpful in bolstering security in the Persian Gulf. He will also want to sound out the new U.S. administration on the huge natural gas deal under negotiation between the Soviet Union and a number of West European countries. The project involves construction of a pipeline from Siberia across Europe that would be financed and built largely with West German money and materials.

On El Salvador, Genscher will try to convince his U.S. hosts of the need for a political solution between the civilian-military government supported by the United States and elements of the opposition Democratic Revolutionary Front that have strong ties to Western Europe's Social Democrats.

One still private official concern here is that the Reagan administration may attempt to extend its hard-line anti-Cuban, anticommunist campaign to other parts of the world, particularly to southern Africa. Bonn fears that such a move might endanger the West's efforts to mediate between South Africa and Namibian guerrillas.

Following his U.S. visit, Genscher is due to travel to Warsaw and then to Moscow for top-level talks -- missions that could enable the West German minister in the end to gauge what common ground is left between the superpowers.