President Trump on Thursday suggested the United States may not defend NATO allies that have not contributed enough to defense spending, casting doubt on a core aspect of the alliance.

“Well, I’ve said that to them,” Trump told reporters in the Oval Office when asked about the potential change in approach. “I said, ‘If you’re not going to pay, we’re not going to defend.’ I said that seven years ago, and because of that, they paid hundreds of billions of dollars.”

Article 5 of the NATO alliance charter states that an attack on one member is an attack on all. The only time it has been invoked was after the Sept. 11, 2001, attacks on U.S. soil.

Asked if he intended to make that official U.S. policy, Trump called it a “commonsense approach,” and he expressed skepticism that other NATO allies would defend the U.S. if it got attacked.

“I think it’s common sense. If they don’t pay, I’m not going to defend them,” Trump said. “I got into a lot of heat when I said that. You said, ‘Oh, he’s violating NATO.’ And you know, the biggest problem I have with NATO … I know the guys very well. They’re friends of mine, but if the United States was in trouble, and we called them, we said, ‘We got a problem. France, we got a problem.’ Couple of others, I won’t mention. Do you think they’re going to come and protect us? They’re supposed to. I’m not so sure.”

Asked why the U.S. would stay in the alliance at all, Trump said he viewed the alliance as “potentially good” but “very unfair.”

The comments echo similar remarks Trump made on the campaign trail last year, which became fodder for Democratic attacks.

Trump’s nominee for NATO ambassador, Matthew Whitaker, told senators at a confirmation hearing this week that the U.S. commitment to the alliance would be “ironclad.”

Trump has long pushed for NATO allies to spend more on defense, arguing the U.S. takes on a heavier burden than other members.

NATO members are each expected to spend 2 percent of their gross domestic product (GDP) on defense, but Trump has pushed for that target to be increased to 5 percent.

The U.S., which spends around 3 percent on defense, would also have to significantly increase the Pentagon budget to reach Trump’s target.

As of last year, 23 out of 32 countries in the alliance had met the 2 percent threshold.

NBC News first reported that Trump was considering a change to how the U.S. approaches the NATO alliance that would favor members that spend more on defense.

NATO has been in the spotlight amid the war in Ukraine, which began more than three years ago when Russian forces invaded their neighbor. Proponents of the alliance have argued that the mutual defense pact is what would prevent further Russian aggression against NATO members like Poland. Sweden and Finland officially joined NATO last year.