Donald Trump said he was "very disappointed" with several of America's Nato allies during a summit in Ankara, while hailing Turkish President Recep Tayyip Erdogan as a "friend".
The US President said he would not have attended the gathering without Mr Erdogan, praising his counterpart for having "gone all out" to host the summit in the Turkish capital.
He also sad he was "very disappointed" with Nato because its members failed to help the US in the Iran war.
One senior military source from a European country said Mr Trump sounded like a "broken record".
Nato allies showcased a series of military projects worth at least $37billion at the summit in an attempt to prove they were committed to defence.
Among the military project announced were weapons to bolster air defences, a range of missiles and other munitions.
The alliance has set a 3.5 per cent of GDP target to be spent on core defence spending by 2035, with Britain placed dead last in overall Nato rankings - spending only 2.6 per cent of national output.
At a joint press conference, the President heaped praised on to Mr Edgogan.
Donald Trump said President Erdogan was a 'friend' who had 'gone all out' to host the summit
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"If it weren't held in Turkey, where my friend happens to be a very strong leader, a very strong person, it's possible that I wouldn’t have attended," he said.
"I thought I had to attend because of the fact that, you know, I know he's gone all out … We weren't treated well because we did something in Iran.
"We don't need anybody's help. I didn’t even want the help. They said they wouldn't be there, and we've invested trillions of dollars in Nato."
Mr Trump was also considering to allow the sale of F-35 jets to Turkey, a reversal of a ban implemented seven years ago.
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Britain is set to announce it will lead a European coalition to manufacture deep precision strike missiles
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The ban was a result of Turkey purchasing the Russian S-400 air defence system.
The President said it was a decision "we're going to make", but there are legal hurdles to put the sale in place.
He also threatened to reignite a diplomatic row with Denmark, again claiming Greenland should be "controlled by the US".
Chancellor Rachel Reeves rebuked the President's remarks.
Donald Trump has also considered resuming the sale of F-35 fighter jets to Turkey
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"The future of Greenland is up to the people of Greenland and of Denmark, and not up to the US president," she said.
"I've been very clear about that ever since it was first suggested."
The UK is set to announce plans to lead a European coalition to manufacture deep precision strike missiles.
Allies will pledge £37billion over the next ten years towards their development - the most advanced weapons in the Nato arsenal.