The US ambassador to Germany has warned of sanctions against firms linked to the Nord Stream 2 gas pipeline from Russia, the American embassy in Berlin confirmed Sunday.
A letter envoy Richard Grenell sent to several businesses "reminds that any company operating in the Russian energy export pipeline sector... is in danger under CAATSA of US sanctions," an embassy spokesman told AFP.
The Countering America's Adversaries Through Sanctions Act (CAATSA) adopted in 2017 targets Iran, Russia and North Korea.
The letter by Grenell, a close ally of President Donald Trump, "is not meant to be a threat, but a clear message of US policy," the spokesman said.
"The only thing that could be considered blackmail in this situation would be the Kremlin having leverage over future gas supplies," he said.
Construction has already begun on Nord Stream 2, set to double the capacity of an existing pipeline across the Baltic Sea.
Alongside Russian giant Gazprom, firms including Germany's Wintershall and Uniper, Dutch-British Shell, France's Engie and Austria's OMV are involved in the project.
Combined with the planned TurkStream pipeline across the Black Sea, Nord Stream 2 would do away with the need to transport natural gas to Europe via Ukraine -- robbing the country of a factor shielding it from Russian aggression, Grenell said.
The two countries are in conflict over the eastern part of Ukraine and the annexation of the Crimean peninsula by Russia in 2014.
"Firms supporting the construction of the two pipelines are actively undermining the security of Ukraine and Europe," Grenell wrote.
Washington's fears about the pipeline are shared by a number of eastern European Union countries including Poland, and the European Parliament last month passed a resolution condemning the construction.
But German Chancellor Angela Merkel, backed by France and Austria, has so far insisted it is a "purely economic project" that will ensure cheaper, more reliable gas supplies.
The veteran leader -- a key player in Moscow-Kiev peace talks -- says Ukrainian interests will be protected as some Russian gas will still be transported via the country once Nord Stream 2 is online.
German Foreign Minister Heiko Maas also weighed in on the transatlantic row last week, saying "European energy policy should be decided in Europe, not in the United States."
Even the opposition Left party condemned Grenell's intervention, with deputy parliamentary leader Fabio de Masi accusing him of behaving like the "governor in Germany of an emperor from Washington".
He urged the government to summon Grenell for a reprimand.