Ankara, Turkey - President Donald Trump offered NATO allies an unexpectedly warm embrace as they wrapped up a key summit Wednesday after earlier lashing out at them over their response to his war on Iran.

It was an abrupt swing from antagonism to affection within the space of a few short hours, illustrating the wide range of emotions exhibited by the mercurial US leader.

"It was a great meeting, there was a lot of love in that room, a lot of unity," Trump told reporters after the closed-door meeting of 32 heads of state at the NATO summit in the Turkish capital, Ankara.

Behind closed doors, Trump had reassured them he wanted the US to stay in the military alliance, saying: "We want to remain with you," a source inside the session told AFP.

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And that was reflected in the final declaration in which NATO leaders reaffirmed their "ironclad commitment" to the mutual assistance clause enshrined in Article 5 of the alliance treaty.

"An attack on one is an attack on all," it said, in wording that sought to ease concerns about Washington's commitment to the alliance.

But the day had not begun well with Trump lashing out over NATO allies' failure to back his Iran campaign just before the session, threatening to cut trade with Spain, and insisting he still wants NATO member Denmark's territory of Greenland.

"I'm very upset with NATO... because of what they did with Greenland, and... because of the fact that they didn't want to help us with the number one state sponsor of terror, that's Iran," he said.

The summit comes at a fraught time for the 77-year-old transatlantic alliance, with Trump demanding members make good on a pledge to ramp up defense spending as Washington takes a step back from Europe.

Keen to avoid a new confrontation with Trump, NATO allies unveiled tens of billions in new arms contracts on Tuesday in a bid to prove they were making good on a pledge to hike defense spending.

NATO chief Mark Rutte insisted the alliance was emerging stronger from the summit in Turkey – despite the disagreements.

"I always felt that families where sometimes you have a heart-to-heart, and sometimes you fight each other a bit, are much stronger," he said.