The Reagan administration, trying to delay and disrupt the Siberian natural gas pipeline under construction to western Europe, is also considering ways to stop a second, parallel Soviet pipeline from getting started.

The second line will be on the drawing boards for several more years, and some U.S. and European specialists dispute whether it will ever be built. These specialists cite forecasts of a slowing growth rate in demand for natural gas, the prospects for greater production from such western countries as Norway and the Netherlands and the lack of open discussion in Europe and the Soviet Union recently about a second line.

But administration officials believe it is "very likely" that the "second strand" of the Siberia-to-Europe gas transmission system remains in Moscow's long-range plans, which call for twin lines to export the Soviets' vast natural gas reserves.

In this view, the "second strand" remains a potential target for some officials here intent on imposing economic pressure on the Kremlin as a way to crimp Soviet military and foreign expansion.

In addition, the second gas line, if it gets into the serious planning stages, represents an opportunity for Washington to impose an economic sanction on Moscow that might be supported by America's western European allies. Agreement on such a plan by the allies could become part of a trade-off within the NATO alliance in return for "some give," as U.S. officials put it, in the Reagan administration's sanctions against the pipeline already under construction.

The administration created a major confrontation with the European allies with its embargo against use of U.S. technology by foreign firms or subsidiaries of U.S. firms in construction of the first pipeline. Aside from being potential recipients of the Soviet gas, the Europeans are supplying much of the equipment, technology and credit for building the pipeline. Firms in France, Italy, England and West Germany are defying the Reagan order, arguing their contracts were signed before the U.S. embargo and must be fulfilled under their laws.

There are no contracts, however, for the second pipeline. So some officials here believe there is a better chance for European agreement on sanctions against that project.

The second pipeline, which specialists say was last discussed openly in the Soviet Union last year, would tap the Yamburg gas deposits roughly about 150 miles north of the Urengoy fields that will feed the pipeline now being built.

This second strand would parallel the first along the 3,000-mile transmission system connecting the Arctic region of western Siberia to the western borders of the Soviet Union, from where the system eventually could serve 10 European countries. The initial line probably will begin operating in the mid-1980s barring further complications. The Soviets have pushed full development of the Yamburg field out of the current 1980-1985 five-year plan and into the future, which contributes to the view of some officials and experts that the second line may never be built. Other officials, however, believe the Soviets will go ahead and that Washington needs to adopt the long-range view.

"The Soviets need money, and they are sitting on the world's biggest reserves of natural gas and, eventually, they are going to exploit it," one official argues. He thinks Moscow could undercut world market prices if necessary to attract customers.

The administration has argued that the current pipeline deal could make western Europe too dependent on a potential adversary for a major source of energy and that export of the gas pumps billions into Soviet coffers for further military expansion.

In discussing the second pipeline, officials say their opposition is not linked to the imposition of martial law in Poland -- the stated reason for the U.S. embargo on the first pipeline -- but to longer term economic policy toward Moscow.

U.S. officials also would like allied agreement on a plan that would stop foreign governments from providing relatively easy credit terms and below-market interest rates to Moscow and would tighten curbs on technology transfer.

An alliance agreement not to become involved in the second pipeline would fall into this category of possible future steps.