Trump directly scolds NATO allies, says they owe 'massive' sums

US President Donald Trump on Thursday intensified his accusations that NATO allies were not spending enough on defence and warned of more attacks like this week's 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 -- even though allied contributions are voluntary, with multiple budgets.

His scripted comments contrasted with NATO's choreographed efforts to play up the West's unity by inviting Trump to unveil a memorial to the September 11, 2001, attacks on the United States at the new NATO headquarters building in Brussels.

"Terrorism must be stopped in its tracks, or the horror you saw in Manchester and so many other places will continue forever," Trump said, referring to Monday's suicide bombing in the English city that killed 22 people, including children.

"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.

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."

In another unexpected twist, Trump called on NATO, an organisation founded on collective defence against the Soviet threat, to include limiting immigration in its tasks.

And Trump did say that the United States "will never forsake the friends who stood by our side" but NATO leaders had hoped he would more explicitly support the mutual defence rules of a military alliance's he called "obsolete" during his campaign.

Instead, he returned to a grievance about Europe's drop in defence spending since the end of the Cold War and failed to publicly commit to NATO's founding Article V rule which stipulates that an attack on one ally is an attack against all.

"Twenty-three of the 28 member nations are still not paying what they should be paying for their defence," Trump said, standing by a piece of the wreckage of the Twin Towers.

"This is not fair to the people and taxpayers of the United States, and many of these nations owe massive amounts of money from past years," Trump said as the other leaders watched.

Nicholas Burns, a former long-time diplomat and ambassador to NATO from 2001-2005, now a professor at Harvard's Kennedy School of Government, said every US president since Harry Truman had pledged support for Article V and that the United States would defend Europe.

White House spokesman Sean Spicer said Trump was "100 percent" committed to collective defence. "We are not playing cutesie with this. He is fully committed," Spicer said.

Praise was always going to be in short supply after Trump's sharp election campaign criticism of the alliance, which he blamed for not doing more to combat terrorism.

Last year, Trump threatened to abandon US allies in Europe if they did not spend enough on defense, comments that were particularly unnerving for the ex-Soviet Baltic states on Russia's border which fear Moscow might try a repeat of its 2014 annexation of Ukraine's Crimea.

Although he has since softened his tone in phone calls and meetings with Western leaders, Trump's sharp words on Thursday recalled his awkward meeting with Angela Merkel in March, when he pressed the German chancellor for Germany to meet NATO's military spending target.

NATO diplomats planned to placate Trump with a pledge on Thursday to agree to national plans by the end of this year showing how NATO allies will meet a promise to spend 2 percent of economic output every year on defense by 2024.

But Trump increased the pressure, calling that agreement made at a summit in Wales in 2014 "the bare minimum".

"Even 2 percent of GDP is insufficient ... 2 percent is the bare minimum for confronting today's very real and very vicious threats," Trump said.

He also made his presence felt at his first NATO summit, literally, pushing his way past Montenegro's prime minister, Dusko Markovic, whose country joins the organisation next month, in footage that went viral.

Spicer said he had not seen the video but assumed the U.S. president was moving to his designated spot.

NATO nonetheless strived to impress Trump with allied jets flying overhead and a walk through the new glass headquarters, which replaces a 1960s prefab structure.

Trump, a real estate magnate, called the building "beautiful" and joked that he did not dare ask how much it cost.