By blocking the export of specialized parts made by General Electric, the United States appears to have blocked--at least temporarily--the multibillion-dollar, natural-gas pipeline construction project between the Soviet Union and Western Europe.

The pipeline project, viewed as extremely important by West European governments, has been regarded skeptically by U.S. officials from its inception, even before the Reagan administration decided to impose sanctions on the Soviet Union in response to the declaration of martial law in Poland.

Two possible substitutes for GE's participation, both involving a switch to European manufacturers, have been cited by industry experts. The alternatives, however, present a number of uncertainties and risks for the project and would also make it difficult for European governments to hold firm on their pledge not to allow moves that would undercut U.S. sanctions.

West German Chancellor Helmut Schmidt, whose country has a major share in financing and building the pipeline and has contracted to receive a quarter of the annual 40 billion cubic meters of gas scheduled to start flowing in 1984, said in Washington last week that he remained firmly in favor of the deal.

French and Italian officials have sounded somewhat less definite, indicating that they would review their planned participation in the $15 billion project. But all six of the West European nations initially involved in the deal eventually are expected to go ahead with it.

In answer to U.S. objections that the Siberian connection will make Western Europe more vulnerable to Soviet pressure, European officials argue that no excessive dependence will result and that Soviet gas offers a cheap and necessary offset for West Europe's overdependence on oil from the Persian Gulf.

It is still unclear whether the Reagan administration will be open to some kind of accommodation with the West Europeans that might allow the pipeline to proceed without violating the intent of the U.S. sanctions. Alternatively, Washington may decide to turn the pipeline into a test of West European loyalty to the United States. U.S. diplomats in several European capitals where the project is being monitored said Tuesday they have received no instructions from Washington on this point.

West German Economics Minister Otto Lambsdorff said yesterday that a West German pledge not to undermine U.S. economic sanctions against the Soviet Union over Poland applied only to contracts where American firms were the main contractors. Bonn government spokesman Kurt Becker quoted Lambsdorff as telling a Cabinet meeting that the pledge, contained in declarations by the European Community and the North Atlantic Treaty Organization, did not cover deals where U.S. firms were subcontractors.

Other German sources said General Electric is considered a "sub-subcontractor" on the pipeline. Asked whether Bonn's interpretation of the promise not to undermine the sanctions was in agreement with other NATO allies, Becker said: "It was not an off-the-cuff remark."

At issue for European engineers is where to turn for the rotor vanes and other components that GE was to supply for the 125 gas turbines to be used along the 3,500-mile pipeline.

The turbines are to be assembled by three European concerns--AEG Telefunken of West Germany, John Brown and Co. of Britain and Nuovo Pignone of Italy. The rotors, a key component of the turbines, are a GE specialty based on an advanced metallurgical process.

The French firm Alsthom-Atlantique holds a GE license to make the rotors and has been mentioned as an alternative source. But a spokesman for the firm, Dominique de Causans, said yesterday it would be "technically impossible," in view of the company's limited "single-plant" capacity, to fill in for General Electric.

The company won an order last December to supply the Soviet Union with 40 GE-type rotors, the exact use for which de Causans said was not known. This and other contracts, the spokesman said, would tie up the French firm's production base until 1985.

A second option would be to select a different turbine altogether, specifically, one made by Britain's Rolls-Royce, which originally bid against the GE-style turbine for the pipeline.

Using the kind of jet engines that power Boeing's 747 and the European Airbus, the British firm has developed a gas turbine used along the Alaskan pipeline and connections in the North Sea, Canada, the Middle East and the Soviet Union. But its model is considerably lighter than the heavy-duty GE one and could be more complicated and expensive for the Soviets to maintain.

A spokesman for Rolls-Royce, Richard Catling, said the company has not been approached by anyone about substituting turbines.

A spokesman for Telefunken, Friedrich Bender, said company officials were consulting with GE to clarify the application of the U.S. sanctions. Jeremy Wyatt, a spokesman for John Brown, said his company is proceeding with production of the turbines it had contracted for, on the understanding that GE has not actually stopped production of its rotors.

This would suggest an element of hope that the U.S. sanctions will be lifted in time to permit GE's participation. Wyatt said the delivery of the first finished turbines was not scheduled until this autumn.

Another result of the American sanctions--the denial of an export license for pipe-laying equipment manufactured by the Caterpillar Co.--was said by diplomats not to pose as serious a technological obstacle for the pipeline, since the Soviets have some such equipment already.

One Western expert noted that several of the less publicized aspects of the project involve U.S. technology that would be difficult for the Soviets to find elsewhere, although it is not known whether contracts have already been awarded for these. They include special, low-temperature welding equipment, material for wrapping the pipeline to guard against freezing and equipment for X-raying welds before the lines of pipe are buried.