President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE slammed Germany early Wednesday over a gas pipeline deal with Russia, claiming the agreement has made Berlin "captive to Russia" and urging NATO to assess the situation.

Trump took aim at one of America's closest allies within hours of his arrival at the NATO summit in Brussels, setting up what is expected to be an uneasy two days of meetings. He continued his long-running criticism of alliance members for what he views as a failure to contribute enough to defense spending before launching a new line of attack against Germany.

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"I have to say, I think it's very sad when Germany makes a massive oil and gas deal with Russia, where you're supposed to be guarding against Russia, and Germany goes out and pays billions and billions of dollars a year to Russia," Trump said during a meeting with the NATO secretary-general.

"If you look at it, Germany is a captive of Russia because they supply," he continued. "They got rid of their coal plants. They got rid of their nuclear. They're getting so much of the oil and gas from Russia. I think it's something that NATO has to look at. I think it's very inappropriate."

The president was likely referencing the Nord Stream 2 pipeline that would deliver gas from Russia to parts of northern Germany. The deal has drawn criticism from the U.S. and some other European nations, because they fear it would give Russian leverage over Western Europe via energy supply, according to The Associated Press.

On Wednesday, NATO Secretary-General Jens Stoltenberg pushed back against Trump's criticism, urging unity among member nations.

"You know, NATO is an alliance of 29 nations, and there are sometimes differences and different views, and also some disagreements. And the gas pipeline from Russia to Germany is one issue where allies disagree," Stoltenberg said.

"But the strength of NATO is that despite these differences, we have always been able to unite around our core task, to protect and defend each other, because we understand that we are stronger together than apart," he added.

His appeal seemed insufficient for Trump, however.

"How can you be together when a country is getting its energy from the person you want protection against?" the president asked, asserting that Germany is "just making Russia richer."

Trump has drawn criticism at times for his friendly rhetoric toward Russia and its president, Vladimir Putin, with whom he will meet in Finland on Monday.

The tense exchange set the stage for the remainder of Trump's visit. The president has clashed with allies over issues of trade and defense spending in recent months, and he spent the days leading up to the NATO summit trashing European countries for not taking on more of a spending burden.

Trump told reporters during a meeting with Merkel later Wednesday that the U.S. has a "tremendous" relationship with Germany. Trump said the two leaders were discussing defense spending and trade.

Merkel, meanwhile, said that German was a "good partner" of the U.S., adding that they discussed migration and trade.

Updated at 10:19 a.m.