Leaders were convening for a summit in the capital Ankara on Tuesday, hoping to project unity after another bruising year, in which the Iran war once again exposed cracks in the alliance that has underpinned Western security since the end of World War Two.
In a meeting with President Tayyip Erdogan, Trump said he might have boycotted the NATO summit altogether had it not been for his warm relations with the Turkish leader, and did not rule out further troop withdrawals from Europe.
"Well, we're going to see. I was very disappointed with NATO," he said, singling out Britain, France, Germany and Italy for not doing enough to support the US war on Iran.
Trump added that "we weren't treated well" by the allies, even as he reiterated that he did not want or need their help.
"Before I asked, they said they wouldn't be there, and we've invested trillions of dollars in NATO," Trump said.
Trump said he had spoken to Russian President Vladimir Putin and Ukrainian President Volodymyr Zelenskiy before the summit about ending the war that started in February 2022 when Russia invaded its neighbour.
"I think they both want to make a deal. It's too bad it took so long ... Something's going to come out," he said.
While sharply criticising longstanding allies, Trump announced Washington would lift sanctions on Turkey that were imposed in 2020 over Ankara's purchase of Russian air defence missiles. He also expressed a willingness to sell Turkey F-35 fighter jets.
The move would be a major gesture to Erdogan and remove a longstanding irritant in bilateral ties.
NATO members have repeatedly tried to show Trump that they are stepping up. NATO Secretary General Mark Rutte said on Monday Europeans had made "staggering" increases in defence spending.
Before Trump's arrival, Rutte trumpeted a series of initiatives and deals at a defence industry forum, and called for a defence industry "revolution" across the alliance, warning over Russia's massive military spending as well as China, North Korea and Iran.
"We don't have the luxury of time. We need capabilities now to ensure we remain ready. The security situation demands it," Rutte said.
"The hum of machinery must become a roar."
The deals, estimated to be worth at least $US50 billion ($A72 billion) according to one NATO official, included European countries buying surveillance drones from US company Northrop Grumman, and NATO buying planes from Sweden's Saab. Saab shares at one point rose more than five per cent as investors bet on the company benefiting from European rearmament. Morgan Stanley upgraded the stock.
Europe's defence sector has often been criticised as fragmented and saddled with red tape and rivalries between companies and countries. That has left Europe more reliant on purchases of US weapons.
Weak economic growth and the need to maintain generous state welfare provisions have also made defence spending a tougher sell in Europe.
Tensions within NATO, already strained over Ukraine and Trump's desire to wrest Greenland from fellow NATO member Denmark, have deepened since the US attacked Iran in February. Trump has repeatedly criticised NATO members for insufficient support in the conflict, threatening to quit the alliance.
On Tuesday, he renewed his push to wrest Greenland from Denmark.
"That should be controlled by the United States, not by Denmark," he said.
"That's what hurt my relationship with NATO, because Greenland doesn't help Denmark. Denmark doesn't spend money to really help Greenland, but it's an important part for the United States."
