West German Foreign Minister Hans-Dietrich Genscher made clear yesterday he expects construction of the controversial natural gas pipeline between the Soviet Union and Western Europe to go ahead and that continued efforts to disrupt the project, as some in Washington favor, would disrupt the Atlantic alliance instead.

Going ahead with a deal that has already been agreed upon, Genscher said, "is in line with the common conviction and views of all partners of the western alliance that contracts that have been concluded should also be honored."

Genscher, who ended a two-day visit here yesterday, made his remarks on the White House lawn after a 35-minute meeting with President Reagan.

The Bonn minister was accompanied by Secretary of State Alexander M. Haig Jr., who used the occasion to tell reporters that Reagan would probably expand his trip to Europe in June to include a visit to West Berlin.

The Siberia-to-Western Europe pipeline, a $10 billion, 3,600-mile project and the largest East-West undertaking ever attempted, continues to be a source of concern here, but administration officials also said that the president sought to impress upon Genscher how strongly he feels about the situation in El Salvador and about the need for understanding and support of the allies.

Reagan reportedly told Genscher that while the United States understands the underlying economic and social ills that cause turmoil in Central America, there was no doubt in the White House that this was an externally supported revolution that posed dangers for the West. Genscher, according to officials, said he recognized the problem and is known to feel that Western Europe, as a whole, needs to get more involved financially and economically in the region as a step toward political stability.

On the other hand, West European officials say they are worried that U.S. policies in Central America may be too strident and further damage East-West relations in Europe. Similarly, they say it would be unfortunate if U.S. actions in Central America wound up injuring U.S. prestige and relations with Third World and so-called nonaligned nations at a time when there is growing resentment in many of those countries of Soviet actions.

Earlier in the day, senior West German officials who asked not to be identified said that they did not know if the Reagan administration had made a decision on whether to make further efforts to disrupt or delay the pipeline deal. But they said they were convinced that the contract, which involves several key European allies, "is being viewed as a reality" here and that it would be "strange" if Washington allowed it to cause a break in the alliance.

The White House announced last week--and officials reaffirmed yesterday--that it would not take any new steps against the pipeline until Undersecretary of State James L. Buckley returns from Europe, where he is to discuss possible further sanctions against Moscow involving either the pipeline or restrictions on credits. Buckley leaves later this week.

During his brief public appearance yesterday with Genscher, Haig also appeared to acknowledge that the pipeline was a reality.

Haig said that, as always, his talks with Genscher showed the great degree of "convergence" between the two allies on major issues. Asked if that included the pipeline, Haig said, "I would say including the pipeline," at least "from the standpoint of our current policies." He quickly added that "we have always been in opposition to that pipeline" since the issue became a major one in this country last year but also noted that "the Germans have been committed to it for a number of years."

Haig also said he thought allied policies would tend to "converge" during the forthcoming Buckley visit. The United States wants the allies to cut back credits and stop credit guarantees. West German officials said it was possible some restrictions could be made on the length of credits extended to the Soviets and the currency in which credits were given.

But they suggested the idea might better be taken up by the Organization for Economic Cooperation and Development, a group of some two dozen nations that could prove unwieldy and unwilling to provide a de facto endorsement of economic sanctions against Moscow for its support of martial law in Poland.