U.S. President Donald Trump decried the "savage" Manchester suicide bombing as he bluntly badgered NATO leaders to pay their fair share of defence costs to stop global terrorism in its tracks.

Mr. Trump, attending his first formal summit as President, used an unveiling of a Sept. 11, 2001, memorial at the new NATO headquarters in Brussels as the backdrop Thursday to scold fellow alliance leaders to forcefully take on Islamist extremists behind bombings in Manchester and elsewhere.

"It was a barbaric and vicious attack upon our civilization," Mr. Trump said. "All people who cherish life must unite behind finding, exposing and removing these killers and extremists."

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Opinion: Canada doesn't deserve its reputation as a defence laggard

British Prime Minister Theresa May complained to Mr. Trump about a stream of U.S. leaks to American media of crucial intelligence about the Manchester attack, which included the publication of forensic photographs of the bomb site by The New York Times. "I will make clear to President Trump that intelligence that is shared between our law-enforcement agencies must remain secure," Ms. May said in a statement.

Many of the North Atlantic Treaty Organization leaders were grim-faced during Mr. Trump's remarks, although Prime Minister Justin Trudeau showed no emotion as the President railed at them for failing to spend 2 per cent of economic output on defence.

"Two per cent is the bare minimum for confronting today's very real and very vicious threats," Mr. Trump said.

Only five of the 28 alliance states meet that spending target, with Canada in a three-way tie for 20th place.

"NATO members must finally contribute their fair share and meet their financial obligations. But 23 of 28 members are not paying what they should be paying and what they are supposed to be paying for their defence," the President said.

Mr. Trump also took a swipe at NATO for spending $1.2-billion (U.S.) on new headquarters in the Belgium capital.

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"I never asked once what the new NATO headquarters cost. I refuse to do so," he said. Later he pushed aside Montenegro's Prime Minister Dusko Markovic to get in front for the official leaders photo.

Mr. Trump declined to reiterate a long-standing U.S. commitment to the alliance's mutual pledge to defend any member under attack. He had first cast doubt on the provision during the presidential election. The alliance has only once invoked Article 5 – the day after the 9/11 terrorist attacks, which triggered NATO's participation in the war in Afghanistan.

He did, however, offer some solace to NATO leaders who are anxious about the U.S. investigation into possible collusion between Mr. Trump's campaign and the Kremlin, saying the alliance must be vigilant to Russian threats in Eastern Europe and the Baltics.

Even before the British atrocity, Mr. Trump had drawn up a NATO shopping list, including a call to boost defence spending, join the U.S.-led coalition in Iraq and Syria and dispatch more military trainers to Afghanistan to counter the rise of the Taliban.

Mr. Trudeau came to Brussels armed with a ready response to Mr. Trump's call for more money and troops, promising to unveil a new defence policy on June 7. He argued Canada does more than its fair share, even though military spending accounts for only 1 per cent of GDP.

"Canada has always been recognized as one of the go-to partners in NATO, a country that consistently steps up and steps forward and delivers on the capabilities internationally for NATO operations," Mr. Trudeau told reporters.

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At the dinner wrapping up the summit on Thursday evening, Mr. Trudeau's communications director, Kate Purchase, said the Prime Minister made a "very passionate" plea for NATO unity.

"He talked about it kind of being the club of the good guys," she said. "He talked about the need for unity around the table and why it was important for every country to step up whether it is through spending or whether it was through contributions or capabilities." Ms. Purchase said the Mr. Trump gave Mr. Trudeau a thumbs-up and other leaders loudly applauded his remarks.

In this remarks earlier in the day, Mr. Trudeau maintained Canada is already playing an active role in the fight against Islamic terrorism and doesn't plan to send more troops to Iraq or recommit Canadian forces to Afghanistan, a war that claimed the lives of 161 Canadians.

"We served and did a tremendous amount in Afghanistan for a decade. … We have no troops in Afghanistan at this time, but we are happy to be supporting in other ways," he said.

Mr. Trudeau also played down Canada's decision to quietly withdraw one of two military surveillance aircraft from Iraq.

"There was nothing surprising or sudden about this," he said. "This is simply part of a regular rotation that was foreseen, that other countries were going to step in after a normal rotation by Canadians."

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He noted about 200 Canadian special forces are in Iraq, CF-18 fighter jets are part of a NATO reconnaissance force in Iceland and the country is heading up a 1,200-member NATO mission in Latvia.

With Manchester on the minds of all the leaders, Mr. Trudeau said one of the biggest roles Canada plays in the war against terrorism is the intelligence picked up by Canada's ultrasecret eavesdropping agency, known as the Communications Security Establishment.

"I am not going to go into details but there are many, many occasions upon which we have directly participated and other occasions benefited from information sharing between security agencies and at the highest levels," he said.

He would not say if Canada had provided help to the British in the hunt for the terrorists behind the Manchester bombing.

Nor would the Prime Minister say whether he had any concerns about Mr. Trump sharing highly classified intelligence – reportedly from Israel – with Russian Foreign Minister Sergey Lavrov during a meeting in the Oval Office earlier this month.

When pressed on the intelligence leaks about which Ms. May complained to Mr. Trump, Mr. Trudeau would only say that Canada and its allies would continue to share intelligence with the United States, which has the world's most sophisticated spying and eavesdropping operation.

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"The track record has shown that collaboration and co-operation between allies, friends and partners has saved lives and keeps all of our citizens safe," he said.

Hours before the summit officially started, NATO Secretary-General Jens Stoltenberg announced that the military alliance will join the U.S.-led coalition fighting the Islamic State in Syria and Iraq.

"This will send a strong political message of NATO's commitment to the fight against terrorism," Mr. Stoltenberg told a news conference. "But it does not mean that NATO will engage in combat operations."

Germany, France and Italy had concerns about NATO formally joining the coalition, for fear it could drag the alliance into a ground war and strain relations with Middle East countries.

Mr. Stoltenberg said NATO would expand the role of its AWACS surveillance planes and air-to-air refuelling for coalition operations, as well as stepping up training of Iraqi forces.He also announced that a special terrorism centre will be set up at the new NATO headquarters to co-ordinate anti-terror intelligence and planning.