The U.S. ambassador to Germany warned several companies that they could face sanctions if they continue their involvement in construction of a Russian-led pipeline, Reuters reported Sunday.

According to the news service, Ambassador Richard Grenell wrote to German companies Uniper and Wintershall to remind them that "any company operating in the Russian energy export pipeline sector is in danger under CAATSA of U.S. sanctions." The letter refers to the Countering America’s Adversaries Through Sanctions Act, which was signed in 2017.

A U.S. Embassy spokesman told Reuters that the letter was not intended as a threat, but as a reiteration of U.S. policy.

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A spokesman for Uniper declined to comment to Reuters, and Wintershall did not immediately respond to a request for comment.

The letter pertained to work on the Nord Stream 2 gas pipeline, which would deliver gas from Russia to parts of northern Germany.

The deal has drawn criticism from the U.S. and some other European nations who fear it would give Russia leverage over Western Europe via its energy supply.

Reuters reported that German government officials were caught off-guard by the letter. One diplomat told the news outlet that the U.S. Embassy's approach did not match common diplomatic practice, and that Berlin would address the issue with officials in Washington, D.C.

The Trump administration has seized on development of the pipeline to criticize Germany. President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE last year suggested the agreement made Berlin "captive to Russia," calling the deal "very inappropriate."