Stephen Harper has started to lay out the case for going to war in Iraq after cabinet ministers discussed on Tuesday a proposal to deploy Canadian jet fighters against Islamic State militants.

Conservatives are promising a decision before week's end on whether they will launch a combat mission.

Sources say the Prime Minister will talk to the Conservative caucus Wednesday about a U.S. request for Canada to help with air strikes against Islamic State forces. This could include between five and eight CF-18 fighter aircraft as well as tanker aircraft that conduct in-air refuelling of warplanes.

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Mr. Harper will also discuss with caucus whether to extend a 30-day military advisory mission in northern Iraq, due to end Oct. 4, where nearly 70 special forces soldiers from Canada are helping Kurdish fighters.

The government's often-confrontational tone with opposition MPs changed noticeably in the Commons Tuesday as Mr. Harper called on MPs to "put aside partisanship in this chamber" and consider the international coalition that's signed on to the fight against Islamic State militants – efforts he described as "necessary … and noble actions."

The United States, Britain and France have already launched air strikes against militants and Belgium and Denmark have pledged planes to support this aerial campaign against Islamic State forces that for months wreaked havoc across parts of Syria and Iraq.

"This has a broad range of support from the international community, including not just conservatives but liberals and social democrats the world over," Mr. Harper said of the alliance arrayed against the Islamic State.

"When we think that something is necessary and noble, we don't sit back and say that only other people should do it. The Canadian way is that you do your part."

The Prime Minister's approach on Iraq is markedly different from last week when the Conservative MP who serves as Mr. Harper's chief parliamentary aide refused to answer basic questions in the Commons about the current deployment of military advisers to Iraq.

Mr. Harper pledged to opposition parties Tuesday that any combat deployment, "including aerial combat," would be put to a vote in Parliament.

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The Conservative majority in the Commons will ensure such a motion easily passes but the Tories want Liberal Party support for any Canadian participation in air strikes. Just as with the combat mission in Afghanistan, the Tories would prefer a bipartisan consensus on combat deployment, the sort of political cover that ensures they're not alone in the fight.

The Liberal Party says it's open to supporting a combat mission, despite statements last week by MP Marc Garneau who vowed to oppose air strikes. Liberal Leader Justin Trudeau told reporters he wants to hear the government's case before deciding.

"Before anyone opens the door to a combat mission, we're going to have to have a level of clarity that so far, this government has been completely unable to provide," Mr. Trudeau told reporters Tuesday.

A key cabinet committee was briefed Tuesday by Canada's top general, Chief of the Defence Staff Tom Lawson, as it debated whether to deepen this country's involvement in Iraq.

Conservative MP Laurie Hawn, a former fighter pilot, told reporters he believes the Tory caucus unanimously supports a more robust role in the fight against the Islamic State.

"Canada stands up when standing up needs to be done," Mr. Hawn said.

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Canada has close to 80 CF-18 fighters remaining in its aging fleet of warplanes, and a military source said at any given time about one-third of them can be called upon, with the rest in maintenance or training. The priority job for the CF-18s is the defence of North America, through the North American Aerospace Defence Command. Four fighters are already flying air-policing operations in the Baltic States as part of a North Atlantic Treaty Organization move to blunt Russian aggression in the region.

The Prime Minister said his government's goal is to help allies destroy the Islamic State militants until they can no longer launch what he called genocidal attacks on people in Iraq and Syria and no longer pose a threat to Western nations, including Canada. Militants have called on supporters to kill Canadians and other Westerners.