A senior U.S. diplomat in Europe has warned Washington that recent Soviet activities in the West European gas market could hinder long-term development of Norwegian offshore natural gas fields and make West Germany, France and Italy more dependent on Siberian energy supplies over the next two decades.

Citing reports from Esso, the European branch of Exxon, U.S. Ambassador to France Evan G. Galbraith cabled the State Department last month that unusual gas sales by the Soviets, at below-market prices, "threaten to undercut Norwegian gas prices sufficiently to make it possible for the Soviets to thwart the development of the Norwegian Troll field -- at least for purposes of sales to the continent."

The cable was made available to The Washington Post by Reagan administration officials concerned about a possible softening of the hard-line approach to the Soviets on economic issues. The warnings came as the administration is attempting to get agreement from the European allies to limit their involvement in future Soviet energy projects in order to reduce the flow of capital, technology and hard currency to the Kremlin.

Such an agreement would end months of tension between the United States and its European allies, enable the United States to lift its trade sanctions against the Soviet natural-gas pipeline and provide a preelection demonstration of administration statesmanship.

European governments have unanimously made clear that, regardless of U.S. objections, they will honor contracts to supply technology and equipment for the planned pipeline from western Siberia. They have also indicated that they will stand by commitments to increase imports of Soviet gas beginning in 1984.

But under a compromise being pressed by the administration, the Europeans would promise to accelerate development of western energy resources, such as those in the North Sea, instead of underwriting a second, parallel Soviet pipeline from Yamal in Siberia.

To avoid a "gloomy scenario" in which the Soviet Union locks up the West European natural-gas market for years to come, Galbraith suggested in his cable that the United States urge the major European gas importers, Ruhrgas and Gaz de France, to "commit to the Norwegian Troll field gas on a formula basis as soon as possible."

Although this option clearly has strong political appeal, it is fraught with uncertainties and practical problems, according to Norwegian and U.S. energy specialists.

Neither of the two major untapped Norwegian offshore fields, Sleipner and Troll, has been designated for commercial development, and gas cannot begin flowing from either of them until the 1990s, according to a Norwegian briefing paper issued in August.

In the case of the Troll field, located under more than 1,000 feet of water west of Bergen, access to the estimated 50 trillion cubic feet of reserves will require technologies not yet available and huge amounts of capital.

Adding to the uncertainty are disagreements about future European energy needs that have made gas importers reluctant to sign long-term contracts, at fixed prices, for new and difficult-to-obtain Norwegian gas. But such commitments are needed before commercial exploration of the Norwegian fields can begin.

Esso, source of the information reported by Galbraith, has a major stake in pending European energy decisions. It is a minority shareholder of Ruhrgas, the West German company that imports Siberian gas but also one of two operators of the Sleipner field, along with the Norwegian national oil company Statoil. Norwegian officials said last week that a decision whether to approve the Sleipner area for commercial use is expected soon.

All Norwegian gas that could technically be produced before 1990 has been sold to European buyers, but the Soviet Union has a surplus capacity of 14 billion to 15 billion cubic meters a years in its pipelines to western Europe.

This situation has given the Kremlin considerable leverage in west European gas markets.

Citing Esso sources, Galbraith said in his cable that the Soviets had been making short-term, or "spot," sales for deliveries covering only three to 12 months at below-market prices and that "the Germans have been buying."

This procedure, described by Esso officials as an "unusual phenomenon" in a gas market normally dominated by long-term contracts, could represent a Soviet effort to push out potential competitors, such as Norway. But officials said another motive could be Moscow's urgent need to raise hard currency to buy western grain and equipment and pay old debts.

In recent months, Norwegian leaders have shown growing concern that the country could lose out to the Soviets in the European energy market well beyond the year 2000. In private analyses, Statoil and the Norwegian Petroleum Directorate have stressed the gravity of the situation for Norway's economy.

Galbraith, who declined through a spokesman to comment yesterday, was reported to have visited Ruhrgas recently. In his cable, he said Ruhrgas and Gaz de France would be more likely to commit themselves to purchase new Norwegian gas "if they thought the Siberian pipeline were going to be delayed."

At the same time, Galbraith suggested that U.S. Ambassador to Italy Maxwell M. Rabb use the information generated from Esso sources "in his continuing efforts to demonstrate to the Italians the virtues of not buying the Soviet gas."