WASHINGTON (REUTERS) - US officials have told members of Congress that they have an understanding with Germany about shutting down the Nord Stream 2 natural gas pipeline if Russia invades Ukraine, a senior congressional aide told Reuters on Tuesday (Dec 7).

The White House said Germany had made commitments about the Nord Stream 2 pipeline that runs from Russia to Germany under the Baltic Sea in the event of aggressive acts by Russia.

US President Joe Biden told Russian President Vladimir Putin on Tuesday that the West is concerned Russia will invade Ukraine and warned of "strong economic and other measures" as punishment should Moscow start a military conflict, the White House said.

US National Security Adviser Jake Sullivan said the fact that gas was not yet flowing through Nord Stream 2 - led by Russian gas giant Gazprom - created leverage for the West over Mr Putin.

"If Vladimir Putin wants to see gas flow through that pipeline, he may not want to take the risk of invading Ukraine," Mr Sullivan told reporters at the White House.

Mr Sullivan said the United States had been in touch with the outgoing and incoming German governments about Nord Stream 2 in the context of a potential invasion. He declined to characterise the conversations further.

"The subject of the future of Nord Stream 2 in the context of an invasion of Ukraine by Russia in the coming weeks is a topic of utmost priority. It has been discussed thoroughly," he said.

US officials say they have received assurances from Germany that the pipeline would be turned off, the congressional aide said. But it was unclear if the two sides had agreed on a definition of invasion, the aide said.

A European diplomat told Reuters US officials had made it clear to allies that they would act to sanction the pipeline in the event of an invasion, which could make any German action a moot point.

"If the US imposes (additional) sanctions, it's an academic point, because no one will be able to do business with Nord Stream 2 for fear of running afoul of US sanctions."

German officials told reporters on Tuesday that there was still a process to complete before the pipeline would even start operations.

Mr Biden has long opposed the Russian-German pipeline. The US State Department has sanctioned Russian entities related to it, but not the company behind it, as it has tried to rebuild ties with Germany that deteriorated under Mr Donald Trump's administration.