US President Donald Trump has told NATO Secretary-General Jens Stoltenberg that Germany is "captive" to Russia because it is importing energy from there.
In a heated exchange with the NATO chief ahead of a summit in Brussels on Wednesday, the US president was fiercely critical of German oil and gas imports from Russia.
Trump said it was "very inappropriate" for the US to be paying for European defence from Russia while Germany is supporting gas deals with Moscow.
"They pay billions of dollars to Russia and we have to defend them against Russia," Trump told Stoltenberg at a breakfast meeting.
"Germany as far as I'm concerned is captive to Russia because it's getting so much of its energy from Russia," he said.
Stoltenberg conceded that there was disagreement between NATO allies over a natural gas pipeline deal between Germany and Russia.
Berlin has been supportive of building a second pipeline to transport Russian gas straight to Germany through the Baltic Sea, despite objections from other EU countries that the pipeline would increase the bloc's dependence on Russian energy.
The US has previously warned it could impose sanctions if the project goes ahead.
Addressing reporters after his breakfast with Trump, Stoltenberg said it's not up to NATO to make a decision on the future of the project.
"This is a national decision," the secretary-general said. "It's not for NATO to settle that issue."
Commenting on the earlier exchange with the US commander in chief, Stoltenberg said that Trump has a "very direct language" but that there was fundamental agreement on the need for "fairer burden sharing in the alliance".
Dozens of NATO leaders are set to meet in Brussels on for what is likely to be a stormy summit.
Although the 29-member military alliance's annual meetings have traditionally been fairly by-the-book affairs, expectations are different this year - thanks, in large, to Trump.
The US president has been openly critical of many of NATO's practices, often railing against Washington spending more money on defence than other member states.
Trump's criticism might put further strain on the relationship between the Western allies, especially following last month's turbulent G7 meeting.