Prime Minister Justin Trudeau is defending a new approach to peacekeeping that will hand the United Nations a key role in deciding where to deploy Canadian troops while keeping peacekeeping resources at their lowest levels in decades.

Mr. Trudeau laid out his government's vision on Wednesday at a UN summit in Vancouver, outlining what was billed as a shift back into peacekeeping – long the country's traditional military role – after the Liberals promised last year to provide up to 600 troops and 150 police officers. The announcement said 200 of those troops will be part of a rapid-reaction team, and pledged tactical airlift support and other equipment.

However, the government had no details to offer about where exactly those resources will be headed and warned it could be months before those specifics are worked out. Critics said the government could have started that work well before this week's conference and argued Canada is giving up too much control to the UN.

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Mr. Trudeau insisted what he called a "smart pledges" strategy will ensure Canadian resources end up where they are most needed.

"It will be the UN telling us where we need to be and us working with them to figure out how we can meet those needs," he told a news conference.

"Canada is re-engaged in peacekeeping and we're doing so in a way that is focused on how we can have maximal impact."

Walter Dorn, a peacekeeping expert at the Canadian Forces College in Toronto, said the Trudeau government should have worked with the UN beforehand so the Prime Minister could arrive in Vancouver with a mission already in hand.

"The waiting game continues," said Dr. Dorn, an employee of the Department of National Defence whose teaching position gives him the academic freedom to speak candidly.

Dr. Dorn also said Canada's contribution is at its lowest level since at least 1990 – a situation that the new commitment won't change.

Andrew Scheer, Leader of the Opposition Conservatives, said Mr. Trudeau's "underwhelming" announcement highlights a void in Liberal foreign policy that will end up giving too much power to the United Nations.

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"He's indicating today that he'll let the UN decide what our priorities are and I believe Canadians want our own government to decide that," Mr. Scheer, who was in B.C.'s Lower Mainland campaigning for a coming by-election, said in an interview.

"We can work in a multilateral way and we can work in a consultative way, but Canadians should decide what our foreign-policy principles are."

Mr. Scheer accused Mr. Trudeau of dithering on Canada's UN peacekeeping commitments.

"He's had two years to put a plan together. He's had two years to work with the UN and international organizations and allies to develop something."

Government officials who briefed reporters said Canada and the UN have only just started what could be six to nine months of discussions about when and where those capabilities are needed.

Mr. Scheer has proposed that Canada lead a peacekeeping mission to war-torn eastern Ukraine to help stabilize the conflict between Kiev and Russian-backed belligerents. Canadian officials have said some form of peacekeeping effort there has been a point of discussion among parties on both sides of the conflict but no agreement has been reached.

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Asked how long before Canadian troops could be taking on new missions, Mr. Trudeau said, "That will be determined by the Defence Minister and the UN co-ordinators."

As the summit ended, the UN Under-Secretary for Peacekeeping Operations said the UN will move quickly to decide how to use the assets Canada is offering.

"We have to do this seriously, methodically but, at the same time, there's a strong determination to turn this commitment into action on the ground, " Jean-Pierre Lacroix told a Wednesday evening news conference.

"I am quite confident we will find the right places to allocate the very useful capabilities that are being offered by Canada," said Mr. Lacroix, adding there are needs in most operations.

Atul Khare, UN Under-Secretary General for Field Support, said the UN is in advanced negotiations with Canada for a C-130 transport plane that would be placed in his department.

Such a plane, he said, could service seven peacekeeping missions including needs in Mali, the Democratic Republic of Congo, the Central African Republic, South Sudan and Darfur.

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"This is the best deployment which we can see," he said, adding many contributions could be used within days or weeks.

Wednesday's announcement also included $21-million to bolster the number of female peacekeepers, acting on an emerging view that gender equity can break down barriers in the field. And Mr. Trudeau announced that retired general Roméo Dallaire will be a partner in a new program to end the recruitment of child soldiers.

Mr. Trudeau also acknowledged the risks of peacekeeping.

"This is something we take very seriously. We recognize there are risks," he said.

More than 80 members of the UN force in Mali have been killed since 2013. A 10,000-member UN force is working in the African country.

"This is the reality of modern peacekeeping, but it's also something that Canadians expect, and the world expects, that we stand up," the Prime Minister said.

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Mr. Trudeau said Canada and the UN are intent on offering better protection for peacekeepers, and Canada will deploy more resources to risky missions.