Brussels, Belgium - US Secretary of State Marco Rubio told nervous NATO members on Thursday that Washington remained committed to the alliance – if they agree to massively ramp up their military spending targets.

President Donald Trump has rattled Europe by casting doubt on his willingness to defend all allies, and by reaching out to Russia over the war in Ukraine – before further raising tensions with his latest trade tariffs.

"Some of this hysteria and hyperbole that I see in the global media and some domestic media in the United States about NATO is unwarranted," Rubio said on his first visit to meet his NATO counterparts in Brussels.

"President Trump's made clear he supports NATO. We're going to remain in NATO," he said.

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Ahead of NATO's June summit in The Hague, Trump has demanded that the alliance more than double its current spending target to 5% of GDP – more than any, including Washington, spend now.

"We do want to leave here with an understanding that we are on a pathway, a realistic pathway to every single one of the members committing and fulfilling a promise to reach up to five percent spending, and that includes the United States," Rubio said.

"No one expects you're gonna be able to do this in one year or two. But the pathway has to be real," Rubio said.

He insisted that Trump was "not against NATO. He is against a NATO that does not have the capabilities that it needs to fulfil the obligations."

A string of European countries have announced steep increases in military budgets, with economic powerhouse Germany opening the way for a major splurge.

"Great things are happening. Over the last couple of months, we literally see hundreds of billions of euros rolling in," said NATO chief Mark Rutte, who backs aggressive militarization.

"So this is probably the biggest increase in defense spending here on the European side of NATO since the end of the Cold War," he said. "But we still need more."

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Officials have said that if Washington is planning a major shift away from Europe and towards confronting China, it needs to agree a clear timeline over years for Europe to fill the gaps left behind.

"There are no plans for them to all of a sudden draw down their presence here in Europe," Rutte said. "But we know that for America, being the superpower they are, they have to attend to more theaters than one. It's only logical that you have that debate."

Europe is nervously eyeing Trump's outreach to Russia as he sidelines allies to press for a deal with Moscow to end the Ukraine war.

Allies are pleading with Trump to stand strong against Moscow as he pushes for a partial ceasefire, despite the warring sides trading accusations of ongoing strikes.