The summit will pledge to boost defense spending in a bid to keep Trump happy, after his return to power in January sparked fears he could blow a hole in the seven decade-old alliance.

But Trump once again raised questions about whether the United States under his administration would honor NATO's core pledge to defend any member nation that is attacked.

"Depends on your definition. There's numerous definitions of Article Five, you know that right?" Trump told reporters aboard Air Force One when asked if he was committed to the mutual defense clause.

"But I'm committed to being their friend. You know, I've become friends with many of those leaders, and I'm committed to helping them."

Trump added that he would discuss the issue further with his NATO counterparts when he arrives in the Dutch city later Tuesday.

"I'm going to give you an exact definition when I get there. I just don't want to do it in the back of an airplane," Trump added.

Trump has repeatedly questioned the founding principle of NATO and even suggested last year that he would encourage Russia to do "whatever the hell it wants" to members that failed to meet his defense spending demands.

Trump meanwhile said he would "take a look" at the proposed language in NATO's final summit declaration on Russia's aggression against Ukraine.

The US president has pivoted towards Moscow as he seeks a peace deal to end Russia's three-year-old invasion of its pro-Western neighbor.

Trump added that he would "probably" meet Ukrainian President Volodymyr Zelensky at the NATO summit, months after the two men rowed in the Oval Office.

"Yeah probably I'll see him," Trump said.

Asked what he would say to Zelensky, Trump replied: "I'll say 'how you doing?' He's in a tough situation, should have never been there."