"It's common sense, right," Trump told reporters in the Oval Office. "If they don't pay, I'm not going to defend them. No, I'm not going to defend them."

Trump said he has been of this view for years and shared it with NATO allies during his 2017-2021 presidential term. Those efforts prompted more spending from other members of the 75-year-old transatlantic alliance, he said, but "even now, it's not enough."

He added: "They should be paying more."

A mutual assistance clause lies at the heart of the NATO alliance, which was formed in 1949 with the primary aim of countering the risk of a Soviet attack on allied territory.

Trump's remarks could trigger alarm bells in capitals from Europe to Asia, where leaders were already worried about a withdrawal of U.S. security support after Trump clashed with Ukrainian President Volodymyr Zelenskiy and showed greater willingness to deal with Russian President Vladimir Putin.

Earlier on Thursday, concerned European leaders backed plans to spend more on defense and pledged to continue to stand by Ukraine.

"I know some may have concerns about NATO's future," NATO Secretary General Mark Rutte said earlier on Thursday. "So let me be clear, the Transatlantic relationship and the Transatlantic partnership remains the bedrock of our Alliance. President Trump has made clear the commitment of the U.S. and his commitment personally to NATO, and it has also made clear the expectation that we in Europe must do more in terms of defense spending."

In the Oval Office, Trump said NATO members were friends of his but questioned whether France or a "couple of others" would protect the United States in a moment of crisis.

"You think they're going to come and protect us? They're supposed to," Trump said. "I'm not so sure."

Trump said he viewed NATO as "potentially good" if what he saw as the spending issue could be fixed. "They're screwing us on trade," he said of the security alliance.

Trump had affirmed the United States' commitments to the mutual defense of NATO as recently as last week during a press conference alongside British Prime Minister Keir Starmer.

At a meeting of European leaders in Brussels, French President Emmanuel Macron responded to Trump's comments by saying France and other European countries had joined U.S. troops in fighting in Afghanistan after 9/11.

"Not only the French, but the Europeans were there when we were called for Afghanistan. And by the way, they weren't politely warned when (the U.S. left Afghanistan)," Macron said. "We are loyal and faithful allies."

A NATO spokesperson referred to Rutte's earlier comments.

Trump also mused on the U.S. treaty alliance with Japan, which he affirmed in a joint statement with Tokyo last month as poised "to grow stronger than ever."

The collective defense arrangement dates back to agreements made after the conclusion of hostilities in World War Two.

"We have a great relationship with Japan, but we have an interesting deal with Japan that we have to protect them, but they don't have to protect us. You know that? That's the way the deal reads," Trump said on Thursday.

"And by the way, they make a fortune with us economically... Who makes these deals?"

The remarks echoed comments Trump made during a 2019 visit to the country.

Japan's embassy did not immediately respond to a request for comment, but Nicholas Szechenyi, a Japan expert at Washington's Center for Strategic and International Studies, called Trump's remarks "technically inaccurate."

While Japan's post-World War Two constitution long restricted its military activities, reforms have meant it could exercise collective self-defense and help protect the United States under certain circumstances, Szechenyi said.

Such circumstances included if an armed attack against a foreign state that is in a close relationship with Japan threatened Japan's survival, he said.