BERLIN (AP) — Three Republican senators have issued a dire warning to operators of a small German port, threatening them with “crushing” sanctions for allegedly providing supplies to Russian vessels involved in a pipeline project the United States vehemently opposes.

The letter sent late Wednesday by Sen. Ted Cruz, Sen. Tom Cotton and Sen. Ron Johnson targets Faehrhafen Sassnitz GmbH, which operates Mukran Port located in German Chancellor Angela Merkel’s constituency on the Baltic Sea island of Ruegen.

The port is a key staging post for ships involved in the construction of the Nord Stream 2 pipeline that's intended to bring natural gas from Russia to Germany.

Last December, Switzerland-based Allseas, which operates ships laying sections of the undersea pipeline, suspended its work after U.S. President Donald Trump signed legislation threatening sanctions against companies linked to the project.

The United States and some Eastern European countries argue that the pipeline will increase Europe's dependence on Russia, a claim both Berlin and Moscow reject.

The three senators say their letter “serves as formal legal notice” that the port operator, its board members, corporate officers, shareholders, and employees risk “crushing legal and economic sanctions” unless they stop providing goods, services and support for the Nord Stream 2 project. This includes providing storage areas for the pipeline's steel sections and provisions for the Russian-flagged vessels Fortuna and Akademik Cherskiy.

“The only responsible course of action is for Faehrhafen Sassnitz GmbH to exercise contractual options that it has available to cease these activities,” the senators add in their letter. It describes the nearly complete pipeline as a “grave threat to European energy security and American national security.”

Officials at Faehrhafen Sassnitz, which is owned by the town of Sassnitz and the state of Mecklenburg Western Pomerania, declined Thursday to comment on the letter.

Merkel's office didn't immediately respond to a request for comment.

Last month, the German chancellor told lawmakers that the U.S. sanctions against companies involved in Nord Stream 2 “don't correspond with our understanding of the law.” She acknowledged that the pressure from Washington had made construction of the pipeline more difficult, but insisted that it was right to press on with the project.

Nord Stream 2, which is owned by Russian gas giant Gazprom, said in a statement Thursday that it is currently seeking alternatives to complete the last 6% of the 1,230-kilometer (764-mile) pipeline following the suspension of work by Allseas.