The United States sees the planned Nord Stream 2 gas pipeline between Russia and Germany as a threat to Europe's energy security, US Secretary of State Rex Tillerson says.
Poland, Ukraine and the Baltic states fear the pipeline would increase Europe's dependence on Russian gas and provide the Kremlin with billions of dollars of additional revenue to finance a further military build-up on EU's borders.
"Like Poland, the United States opposes the Nord Stream 2 pipeline. We see it as undermining Europe's overall energy security and stability," Mr Tillerson said at a joint press conference with the Polish foreign minister in Warsaw on Saturday.
"Our opposition is driven by our mutual strategic interests," he said.
The US has already sanctioned Russian companies over Moscow's involvement in the Ukraine crisis, and foreign companies investing in or helping Russian energy exploration.
Nordic nations have already voiced security concerns over the pipeline being laid near their shores under the Baltic.
But Germany and Austria have focused more on the commercial benefits of having more cheap gas from Russia.
German energy groups Uniper and Wintershall, Austria's OMV, Anglo-Dutch group Shell and France's Engie have invested in the 1225km pipeline, which is to pump more Russian gas via the Baltic Sea.
Poland, which spent four decades under Soviet domination after World War II, still imports Russian gas to meet about two-thirds of its consumption.
But it has invested in a liquefied natural gas terminal on the Baltic coast to diversify supplies and plans to build a gas link to Norway. In November, Poland signed a long-term deal to buy LNG from the US.
"We are proud to support Poland's energy diversification and security," Mr Tillerson said.