U.S. President Donald Trump on Thursday intensified his accusations that NATO allies were not spending enough on defense and warned of more attacks like this week's Manchester bombing unless the alliance did more to stop militants.

Axar.az reports citing Reuters.

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 Sept. 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 organization founded on collective defense 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 defense rules of a military alliance's he called "obsolete" during his campaign.

Instead, he returned to a grievance about Europe's drop in defense 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 defense," 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 U.S. 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 defense. "We are not playing cutesie with this. He is fully committed," Spicer said.