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 on Wednesday wasted no time at the NATO summit in airing grievances over a perceived lack of defense spending by allies, previewing what is likely to be a confrontational next couple of days in Belgium.

Trump started the day by meeting with NATO Secretary-General Jens Stoltenberg, and upon his arrival took credit for an increase in defense spending among alliance members.

However, Trump said that hike is "not nearly enough."

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"Many countries are not paying what they should. And, frankly, many countries owe us a tremendous amount of money for many years back, where they're delinquent, as far as I'm concerned, because the United States has had to pay for them," Trump said.

"So if you go back 10 or 20 years, you'll just add it all up. It's massive amounts of money is owed," he added.

Trump has repeatedly criticized NATO allies for not spending enough on defense, a criticism that has added to tensions between the U.S. and traditional allies such as Germany, France and the United Kingdom that has been further exacerbated by a growing fight over trade.

The U.S. does spend far more on defense than other members of NATO, with U.S. spending representing about 70 percent of the alliance's total.

At the same time, NATO officials have been quick to point out that other countries have increased their spending each year since 2015.

NATO members agreed in 2014 to spend at least 2 percent of their respective gross domestic product on defense by 2024.

This year, seven of the 28 non-U.S. members are meeting or are expected to meet the 2 percent goal, and alliance spending is expected to increase by another 3.78 percent. Additionally, 15 members have plans to meet the 2 percent goal by the 2024 deadline.

The goals are also for each country's own defense budget, not payments into the alliance.

Those efforts have been unsatisfactory to Trump, however, who has for months pushed allies to increase spending and has complained that the U.S. is unfairly burdened with higher costs for little gain.

"You know, we're protecting Germany, we're protecting France. We're protecting everybody. And yet we're paying a lot of money to protect," Trump said.

Trump's opening remarks on Wednesday underscored the president's increasingly hostile rhetoric toward long-time U.S. allies.

In addition to railing against spending discrepancies, Trump claimed Germany is "captive to Russia" because of a gas pipeline deal between the two countries.

"If you look at it, Germany is a captive of Russia because they supply," Trump said during his meeting with Stoltenberg. "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."

Stoltenberg attempted to downplay Trump's concerns and urge unity, while acknowledging that a group as large as NATO is bound to have disagreements.

"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 said.

Trump was unmoved, asserting that the deal was "just making Russia richer."

German Chancellor Angela Merkel also defended the agreement, saying Germany is able to make "independent decisions." She added that Germany is among the largest troop contributors to NATO.

The exchange served as a preview of what is to come for the rest of Trump's stay in Brussels. He is slated to meet one-on-one Wednesday with Merkel and French President Emmanuel Macron. Neither meeting is open to the press.

Trump appears to see his confrontational approach with allies as a political winner with his base, whether it involves tweaking fellow NATO members over their defense spending or imposing tariffs on their exports while arguing the U.S. has been ripped off by international trade deals.

It is a different approach to international politics that is a sharp break from the Obama years, a distinction Trump appears to enjoy.

But it is likely to lead to some uncomfortable scenes during Trump's weeklong trip to Europe, which includes a visit to the United Kingdom, starting Thursday, where protests are expected.

Trump will meet with Prime Minister Theresa May amid upheaval in her own government.

Trump will then head to Helsinki, Finland, for a meeting on Monday with Russian President Vladimir Putin. Trump has raised eyebrows at times with his friendly rhetoric toward Putin, and he suggested before departing for Europe that his meeting with the Russian leader would be his easiest of the week.

"I have NATO, I have the U.K., which is in somewhat turmoil, and I have Putin," Trump told reporters Tuesday.

"Frankly, Putin may be the easiest of them all," he added. "Who would think? Who would think?"