Liberal leader Justin Trudeau reached out to Canada's traditional allies a day after his landslide victory, pledging Tuesday to rebuild ties that soured over the issue of climate change with a "constructive" new style of government.

The decisive win ended nearly a decade of Stephen Harper's Tory rule, giving the 43-year-old Trudeau a strong mandate to change tack on global warming and return to the multilateralism often shunned by his predecessor.

"I want to say to this country's friends all around the world, many of you have worried that Canada has lost its compassionate and constructive voice in the world over the past 10 years," the prime minister-elect told cheering supporters at a rally in Ottawa.

"On behalf of 35 million Canadians, we're back," said Trudeau, the telegenic son of the beloved late premier Pierre Trudeau.

Congratulations poured in from world capitals including Washington, Paris and Beijing, applauding the promised new direction, with all eyes turned towards the Paris climate conference in December.

The White House said President Barack Obama, whose administration had clashed with Harper on climate policy, looked forward to partnering with Trudeau on the issue.

Labeled a "climate laggard" by the UN, Canada under Harper became the first country to pull out of the landmark Kyoto Protocol in 2011, inflicting lasting damage on relations with allies in Europe among others.

Secretary-General Ban Ki-moon urged Canada's new leaders to help clinch a historic climate deal in Paris.

"Canada is a member of the Group of Seven and as a result, it has a particular role to play in terms of providing leadership on climate change issues," UN spokesman Farhan Haq said. "The secretary-general hopes and expects that Canada will play that role."

French President Francois Hollande also voiced hope that Ottawa could make a difference on the climate, in a statement congratulating Trudeau.

"The Paris conference will offer a precious chance to put into action this will to work together for peace and the planet's future," he said.

- New CO2 targets -

Monday's election for many Canadians was a referendum on Harper's autocratic style and their country's loss of clout on the world stage.

As prime minister, Harper radically altered Canada's foreign policy, transforming it from an impartial arbiter to an assertive power with its own agenda, including strident support for Israel and refocusing aid from Africa to South America.

Under Harper the country also lost losing a bid for a rotating UN Security Council seat and failed to get US approval for a pan-continental pipeline first proposed in 2008.

Under Trudeau, "I think we'll see Canada become a more constructive actor in international affairs," University of Ottawa international affairs professor Roland Paris told AFP.

"We're going to see a re-engagement under the Liberals with multilateral organizations like the United Nations, especially with climate talks coming up in Paris" and other summits including the G20 and Commonwealth, echoed professor Lauchlan Munro, also at the University of Ottawa.

Trudeau has said more needs to be done to curb carbon emissions, but he has stopped short of setting specific targets ahead of the Paris talks.

"It's unclear how he would achieve a new target," said Jonathan Paquin, a politics professor at Laval University in Quebec.

Ottawa shares environmental responsibility with its 10 provinces, which are all working to meet different targets. At times this has complicated setting national CO2 reduction targets.

But the issue is already shaping up as a priority for the incoming administration.

Quebec Premier Philippe Couillard said Tuesday "the world wants a change in tone and priorities on the climate file" from Canada.

"We have time before December (for Ottawa and the regions) to come up with a firm target that Canada will pitch at the talks," he said.

- Syrian refugees -

Other foreign policy pledges may prove tougher to deliver on, however.

During the campaign, Trudeau said he would pull Canadian fighter jets from Iraq and Syria where they have been joining in US-led coalition air strikes against the Islamic State group, while promising to keep military trainers in northern Iraq.

Munro said "pressure from allies could be difficult to overcome -- what would the Americans, British and French say if we pulled out?"

Trudeau also vowed to significantly boost Canada's intake of Syrian refugees, more than doubling it to 25,000 by year's end, insisting only political will has been lacking so far. But critics have warned this could be a herculean task.

Canadian foreign aid could also see a bump for the first time since 2010.

The Liberals had criticized a shift from focusing on poorer Africa to South American middle income countries -- and the aligning of aid with Canadian commercial interests.

But Munro noted that the shift in approach began under a previous Liberal administration, in 2004. "It remains to be seen how it will be different," he said.