Canada's fight against Islamic State militants is entering a more complex and dangerous phase under a plan unveiled by Stephen Harper to expand air strikes to Syria.

The Conservative government has tabled a motion seeking the endorsement of the Commons for expanding the Iraq mission and extending the deployment by one year until the end of March, 2016.

The Liberals and the NDP are refusing to support the motion. But it will easily pass because the Tories hold a majority of seats in the House.

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Syria has not explicitly invited Canadian warplanes into its airspace and Prime Minister Stephen Harper had said he would expand the air combat mission only with the "clear support" of the Syrian government.

But on Tuesday, he said he no longer felt it was necessary. "The reality is that the Assad regime does not have the will nor the ability to counter the Islamic State in Syria," he said.

The size of the mission is unchanged. As many as 69 special forces soldiers will remain in northern Iraq training and advising Kurdish fighters and six CF-18 fighters will continue operating out of Kuwait along with two surveillance planes and an aerial refuelling aircraft. The Conservatives continue to insist they will not deploy ground troops in combat.

Canadian warplanes have been bombing Islamic State targets in Iraq since October, but Mr. Harper said Tuesday militants are increasingly seeking safe haven in Syria, where their power base lies, and Canadian jet fighters need to follow them there.

"ISIL's fighters and much of its heavier equipment are moving freely across the Iraqi border into Syria, in part for better protection against our air strikes," Mr. Harper told the Commons, using another name for Islamic State.

Canada will be the only western nation besides the United States conducting coalition air strikes in Syria, where Bashar al-Assad's regime still holds power but is an international pariah because of its war against its civilians.

Debate on the matter begins Thursday and continues Monday with a vote expected Monday night.

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This extension will leave Canadian warplanes and troops deployed in the region during an expected September-October election campaign and the outcome of this federal ballot will determine whether they remain there.

NDP Leader Thomas Mulcair is vowing to end the mission if his party wins office and Liberal Leader Justin Trudeau's officials are saying he'd cancel the combat portion of the deployment and shift to training and humanitarian assistance.

Mr. Mulcair accused the Conservatives of dragging Canadians into a conflict with no long-term plan. "This is not a United Nations mission. This is not a NATO mission," he said. Mr. Trudeau said Canada has a role to play in the region, but added that any military mission must serve Canada's national interests. The Conservative plan "doesn't meet that test," he said.

The expanded mission carries more risk for Canadian Armed Forces and for Mr. Harper. Should a Canadian military aircraft crash in Syria, the pilot or crew would be farther removed from friendly forces than if they were similarly stranded in Iraq.

In December, Islamic State militants took hostage a Jordanian pilot whose plane went down in Syria during coalition air strikes. They later killed Moaz al-Kasaesbeh by burning him alive.

David Perry, senior analyst with the Canadian Defence & Foreign Affairs Institute, said Canadian pilots or air crews could face a longer wait for rescue in Syria.

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"If it's within Iraq, you've got basically half the country that's still Iraqi-government controlled. So presumably the Americans, who are generally the ones doing combat search and rescue, would have faster response times [in Iraq]. … And in Syria, you're dealing with further distance to get there if something happens."

The Department of National Defence declined to discuss how it might rescue pilots or crew in Syria.

It also declined to directly answer whether it believes the mission in Syria is inherently more dangerous than the mission in Iraq.

Defence Minister Jason Kenney doesn't see a greater threat in Syria. "We believe that the risk of operating our air strikes in Syria is modest. There have been no recorded efforts to fire at allied … planes operating over eastern Syria. Frankly, we don't believe the Syrians have the capacity of doing that and neither does ISIL," he told CTV News.

Roland Paris, director of the Centre for International Policy Studies at the University of Ottawa, said he expects Canada will have to make an informal contact with the Assad regime to ensure Canadian warplanes are not targeted by Syria's air defence system.

"We are getting deeper and deeper into a complex regional war with, I think, little sense of where it all leads," Mr. Paris said.

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He said Canada has not adequately justified going to war in Syria under international law.

"To use armed force in Syria, we would have to justify it on the basis of one of three conditions: Either we are there with the express consent with the sovereign government of the country, or we are there with a UN security council resolution, or we are operating in self defence," Mr. Paris said.

"If we believe in a rules-based international order, then we at least have to say how we are operating according to international law."

Opposition parties said humanitarian aid should be a priority, but Mr. Harper insisted Canada could wage war in Iraq and provide humanitarian aid at the same time.

"Among the nations of the world, we have been one of the biggest providers of humanitarian assistance," he said.

"In the last six months, we have helped feed 1.7 million people in Iraq, provide shelter and relief supplies to one and a quarter million people and give some education to at least half a million children," Mr. Harper said.

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"Beyond that, we have also been helping to support more than 200,000 Syrian refugees in Iraq, with food, water, shelter and protection."