The letter was coordinated in Washington by several U.S. government agencies and is not meant to be a threat but a clear message of U.S. policy, the spokesman added.
Russian gas giant Gazprom is implementing the project jointly with its Western partners - Uniper, Wintershall, Engie, Austria's OMV and Anglo-Dutch group Shell.
German Foreign Minister Heiko Maas said on Thursday that any U.S. sanctions against Nord Stream 2 would be the wrong way to solve the dispute and that questions of European energy policy had to be decided in Europe, not in the United States.