U.S. President Barack Obama's final Nuclear Security Summit ended Friday, as Canada pledged to "lead by example" in stemming what is seen as an emerging security threat: the prospect of terrorists armed with nuclear weapons.

Canadian Prime Minister Justin Trudeau, who was among the representatives from more than 50 nations at the summit, noted that Canada was historically one of the original countries involved in the Manhattan Project to produce the first nuclear bombs in the Second World War. But he touted Canada's "deliberate choice" not to become a nuclear weapons state.

"Countries like Canada have to continue to lead by example," he said. "Canada will continue to invest in its world-class nuclear security regime and share our expertise in this field with others."

Two key funding announcements — a new $42-million pledge for bolstering nuclear security internationally, and $51 million in counterterrorism funds drawn from a previously announced pool of money — will go toward that goal.

They were the kinds of concrete commitments that Obama's summit were intended to spur.

"We didn't just come here to talk, but we came here to act," Obama said at the closing session, before laying out inroads the world has made toward removing "the world's most deadly materials from nuclear facilities around the world."

Obama is flanked by Dutch Prime Minister Mark Rutte, left, and Japanese Prime Minister Shinzo Abe as he speaks during the opening plenary session of the Nuclear Security Summit on Friday. (Alex Brandon/Associated Press)

The president hailed the strengthening of treaties and international partnerships, the creation of a new nuclear security contact group of senior-level experts, and the all-out elimination of plutonium and highly enriched uranium from 14 nations and Taiwan since the first Nuclear Security Summit six years ago.

But he acknowledged the evolving nuclear threat remains a pressing concern, one that is far from being neutralized.

"Our work is by no means finished," Obama said. "Nuclear arsenals are expanding in some countries with more small tactical nuclear weapons."

'We are all vulnerable'

Trudeau said it became "very clear" through meetings with other countries that "we are all vulnerable in very similar ways to terrorists using extremely lethal materials, and therefore need to do a better job of sharing and co-ordinating, collaborating in our approach if we are to successfully keep our citizens safe from an increasingly sophisticated network of terrorists."

Trudeau also revealed details of a hypothetical nuclear scenario that world leaders were asked to respond to on Friday, as part of a "credible but also quite alarming" tabletop exercise conducted behind closed doors.

We need to be able to adapt to the reality of modern counterterrorism. - Canadian Prime Minister Justin Trudeau

The discussion involved a theoretical attack involving drones to disperse radioactive material, he said.

"We have to understand that dealing with drones, for example, is something that police forces in the past and security agencies of the past have not had to do," he said. "We need to be able to adapt to the reality of modern counterterrorism."

It was a plausible scenario, he said, but called it more of a "'jumping-off point" for wider discussions rather than a specific breakdown of actions focused on that one scenario. He declined to provide further details.

Asked what may have caught him off guard about the scenario, Trudeau said that simulating reactions in emergency situations reinforced the message that disparate agencies and nations must be better equipped to respond in a cohesive manner to nuclear terrorism.

The more than 50 participating nations at the high-level talks, including Canada, have made some 260 "specific commitments" to date toward enhancing nuclear security, Obama said.

As promising as that progress has been, the U.S. president said, negotiating further nuclear reductions with Russia is one mission that has fallen short. The Russian government, one of the world's biggest atomic powers, boycotted the U.S. summit at a time of heightening tensions between the two nations.

Dealing with North Korea's nuclear problem is another concern, Obama said, adding that he realizes the mission for a world free of all nuclear arsenals "perhaps will not happen in my lifetime."

Leaders took part in a scenario-based policy discussion on Friday focused on a hypothetical nuclear terrorism incident. (Sean Kilpatrick/Canadian Press)

He nevertheless noted that "nuclear arsenals are on track to be the lowest they've been in six decades."

On the Canadian front, Trudeau laid out one such funding pledge on Friday, telling reporters on the floor of a Washington convention centre about a $42-million investment.

The Canadian funds are to be parcelled out over two years. They will include:

$26.5 million for strengthening global efforts for the detection and prevention of illegal trafficking. The funds would help with training and equipment in Mexico, Colombia, Jordan and Peru.

$6.6 million to support the International Atomic Energy Agency's security fund, including programs that support cybersecurity at nuclear facilities.

at nuclear facilities. $5.7 million for enhancing the physical security of nuclear facilities.

$2.3 million to promote security of radioactive sources.

$1 million for projects with the U.S. Department of State to improve transportation security.

$100,000 toward supporting the IAEA with technical expertise and nuclear forensics with the Global Initiative to Combat Nuclear Terrorism.

Trudeau added that Canada would contribute to the fight against ISIS with $51 million in support over three years for counterterrorism efforts in the Middle East and North Africa.

That money will be derived from funding announced in February for programming aimed at stemming the flow of foreign fighters.

During the course of the two-day summit, the prime minister also met with the heads of state of Argentina, Japan and India. Prime Minister Narendra Modi of India has invited Trudeau to India, and he has accepted the invitation, the Prime Minister's Office confirmed, though dates and details are not yet nailed down.

This was the fourth and final nuclear summit, which was initiated by Obama in 2010, and born out of his speech in Prague a year earlier envisioning a nuclear weapons-free world.

Four summits later, the president said Friday, more than a dozen nations have eliminated their stockpiles of highly enriched uranium and plutonium, the materials used in building nuclear and radiological weapons capable of inflicting massive casualties.