Washington (CNN) NATO diplomats are dumbfounded by President Donald Trump's barrage of acidic rhetoric at the annual summit in Brussels on Wednesday.

Trump came out brawling in his first public comments, accusing NATO ally Germany of being "a captive of Russia," calling members of the alliance "delinquent" in their defense spending and insisting they increase it "immediately."

"It's like the world has gone crazy this morning," one senior European diplomat told CNN. "Trump's performance was beyond belief."

The President's remarks sent officials scrambling for answers, triggered ripples of dismay among defense officials and alarmed members of Trump's own party enough that one worried aloud the President is trying to "tear down" the 29-member alliance. The Republican-controlled House, usually careful to stay in lockstep with Trump, passed a resolution to send a "strong message of support" for NATO.

"This is very confusing," another senior European diplomat said. Referring to Trump's targeting of Germany, this envoy said, "the attacks before, and now this tremendous stuff today. It doesn't make any sense. We're still in the process of analyzing it."

'Punch our friends'

Senate Foreign Relations Chairman Bob Corker told CNN he was concerned that that Trump is trying to "tear down" NATO and " punch our friends in the nose ." The Tennessee Republican said he supports the notion of getting NATO countries to increase their defense spending, but he said Trump's rhetoric is "damaging to us."

House Speaker Paul Ryan said that he subscribes "to the view that we should not be criticizing our president while he is overseas, but let me say a couple of things. NATO is indispensable."

Among NATO defense officials, there were quiet questions about how long US Defense Secretary James Mattis -- a former Supreme Allied Commander of NATO and strong believer in alliances -- could stay on in Trump's administration. For the second year, Trump was throwing a tantrum at NATO and Mattis was having to clean up after him.

Publicly, NATO Secretary General Jens Stoltenberg and European leaders pushed back against Trump's blistering attacks on Germany and other partner nations, as they attempted to downplay notions that the alliance may be fracturing.

"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," Stoltenberg told Trump over breakfast.

Other NATO leaders emphasized their public support for the alliance, implicitly pushing back on Trump's attacks.

What good is NATO if Germany is paying Russia billions of dollars for gas and energy? Why are there only 5 out of 29 countries that have met their commitment? The U.S. is paying for Europe's protection, then loses billions on Trade. Must pay 2% of GDP IMMEDIATELY, not by 2025. — Donald J. Trump (@realDonaldTrump) July 11, 2018

British Prime Minister Theresa May used a press conference Wednesday to declare that "NATO is as vital to us today as it ever has been." At a time when Trump's provocative bluster increasingly raises questions about his commitment to NATO, May finished her sentence by declaring that "the UK's commitment to it remains as steadfast as ever."

One NATO diplomat went as far as to credit Trump for spearheading discussions on burden-sharing -- ignoring the President's tendency to misrepresent statistics in order to bolster his argument.

"Allies are spending more thanks to the President," the diplomat said. "Last year was the biggest increase in 25 years."

But public reassurances about the strength of NATO have done little to ease the sense of unease that has been building among the US' closest allies, particularly as Trump continued to single out Germany.

"What good is NATO if Germany is paying Russia billions of dollars for gas and energy? Why are there only 5 out of 29 countries that have met their commitment? The U.S. is paying for Europe's protection, then loses billions on Trade. Must pay 2% of GDP IMMEDIATELY, not by 2025," he tweeted Wednesday.

The new German ambassador to the US conveyed some of the simmering unease in her country and beyond during a Wednesday interview with CNN's Wolf Blitzer, when he asked whether her country thinks Trump would honor Article 5, the alliance commitment to come to a member's aid if they're attacked. The only time it's been invoked was for the US, after the attacks of September 11, 2001.

(LtoR) German Chancellor Angela Merkel, US President Donald Trump and Greek Prime Minister Alexis Tsipras pose for a family picture ahead of the opening ceremony of the NATO (North Atlantic Treaty Organization) summit, at the NATO headquarters in Brussels, on July 11, 2018. (Photo by LUDOVIC MARIN / POOL / AFP) (Photo credit should read LUDOVIC MARIN/AFP/Getty Images)

The second time Blitzer asked Ambassador Emily Haber, she said, "I understand that all NATO members and all NATO countries stand by in a steadfast manner by Article 5." It took a third question from Blitzer - "so that is a yes?" - before Haber responded with a definitive "yes."

"I believe that all NATO members stand by Article 5," Haber said. "No doubt was cast on the credibility of Article 5."

Trump, despite his attacks on Germany, insisted to reporters in Brussels that he and Merkel have a "very, very good relationship."

A third European diplomat said Trump may be targeting Germany and its chancellor in an attempt to win points at home with his base and to deflect attention from his own fawning treatment of Russian President Vladimir Putin ahead of their summit in Helsinki on July 16.

"Targeting Merkel, especially her immigration strategy in the past, is exactly what he wants to attack," the diplomat said, noting that Trump's previous criticism of the German leader's immigration policy has resonated with his own political base at home.

"She has allowed in a lot of immigrants. He's found that these attacks work, and can produce a great deal of damage. Some will make this a big issue," the source added.

'Blaming others'

"Second, the President is under attack himself for the way he treats Putin and the way he may treat Putin in Helsinki," this diplomat said, who suggested Trump's approach was "part of his strategy of blaming others."

The situation only became more complicated later in the day when Trump suggested NATO leaders increase their defense spending to 4% -- doubling the 2% target that many NATO countries have yet to meet.

"President Trump wants to see our allies share more of the burden and at a very minimum meet their already stated obligations," White House press secretary Sarah Sanders confirmed in a statement.

But according to the latest numbers from NATO, the US doesn't even spend as much as Trump is calling on other countries to.

Asked whether Trump had suggested allies raise their defense spending to 4% of GDP, Stoltenberg said, "I will focus on what we have agreed, and we have agreed on 2%, so let's start with that. We have a ways to go, but the good news is that we have started."

'Things can get nasty'

A second senior European diplomat said Trump's 4% suggestion was "not good," highlighting concerns that the conflict over defense spending is fueling perceived cracks within NATO at a time when the alliance should be projecting strength -- particularly ahead of the President's upcoming summit with Putin.

"The summit should send a strong signal to Putin, and not show strong differences between allies. We would prefer a different situation," a third senior European diplomat told CNN.

"But we're not worried about it. We compare it to fights among families. Things can get nasty, but it's still a family," the diplomat added.

A senior NATO diplomatic source said that alliance countries agree there needs to be a push for more defense spending, but warned that the group should be showing greater unity ahead of Trump's meeting with Putin.

But the source added that NATO unity is essential as well, saying it will only "increase the President's negotiating leverage" with Putin. This NATO source suggested that Trump will go into his meeting with Putin with a stronger hand if the NATO alliance is also strong.