Trump berates NATO for dodging defence dues

US President Donald Trump on Thursday intensified his accusations that NATO allies were not spending enough on defence and warned of more attacks similar to the Manchester bombing unless the alliance did more to stop militants.

In unexpectedly abrupt remarks as NATO leaders stood alongside him, Trump said certain member countries owed "massive amounts of money" to the United States and NATO.

Unveiling a memorial to the 9/11 attacks at NATO's new headquarters in Brussels, Trump also urged the alliance to get tougher on tackling terrorism and immigration in the wake of the Manchester attack, which left 22 people dead, including children.

"Terrorism must be stopped in its tracks, or the horror you saw in Manchester and so many other places will continue forever," Trump said.

"These grave security concerns are the same reason that I have been very, very direct ... in saying that NATO members must finally contribute their fair share," Trump said.

Allies who had hoped to hear Trump publicly declare his commitment to NATO's Article 5 collective defence guarantee were left disappointed as he made no mention of it.

"Twenty-three of the 28 member nations are still not paying what they should be paying and what they're supposed to be paying for their defence," the President said as fellow leaders looked on grim faced.

Trump said that even if they met the commitment they made in 2014 to allocate two percent of GDP to defence, it would still not be enough to meet the challenges NATO faces.

"This is not fair to the people and taxpayers of the United States. Many of these nations owe massive amounts of money from past years," Trump added.

NATO Secretary-General Jens Stoltenberg defended Trump, saying that although he was "blunt" he had "a very plain and clear message on the expectations" of allies.

But one senior diplomat said Trump, who left the leaders' dinner before it ended to fly to Italy for Friday's Group of Seven summit, said the remarks did not go down well at all.

"This was not the right place or time," the diplomat said of the very public harangue. "We are left with nothing else but trying to put a brave face on it."