U.S. President Donald Trump's criticism of the Russian-backed Nord Stream 2 pipeline is no reason to stop the project and any attempt to do so would be difficult now that it is being built, European Commissioner Guenther Oettinger said.

Trump has attacked Berlin for supporting the $11 billion gas pipeline spanning the Baltic Sea, accusing Germany in July of being a "captive" of Russia due to its reliance on Russian energy.

U.S. Energy Secretary Rick Perry said last month that Washington retained the option of imposing sanctions on companies working on the pipeline, which would bring Russian gas directly to Germany.

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Berlin and Moscow have been at odds since Russia annexed Crimea four years ago, but they have a common interest in the Nord Stream 2 project, which will double the capacity of the existing Nord Stream 1 route from next year.

"I was never a great supporter of Nord Stream 2," Oettinger, the EU's budget commissioner, told German magazine Der Spiegel. "But the truth is the pipeline has long been under construction and can no longer so easily be stopped."

He added: "Trump's threats are no reason for that."

Germany refuses to join opposition to the project from many EU states and—thus far—from the EU executive, describing it as a private enterprise.

Washington is concerned that the pipeline, which will bypass Ukraine by running under the Baltic Sea, will strip Ukraine of important transit revenue and says Moscow is using the project to divide Europe.

Ukraine derives up to 3% of its gross domestic product from transit charges.

Construction of the project is progressing.

Oettinger, a German, pressed Russian gas giant Gazprom, which is leading the project, to agree "a fair deal on the further use of the existing pipelines through Ukraine."

Gazprom is the sole shareholder in Nord Stream 2, shouldering half of the 9.5 billion euro (US$10.89 billion) construction cost. Gazprom's European partners are Germany's Uniper SE and Wintershall Holding GmbH, Anglo-Dutch group Royal Dutch Shell Plc (NYSE: RDS.A), France's Engie SA and Austria's OMV AG. (US$1 = 0.8720 euros)