Opposition MPs are expressing concerns about the government's handling of Canada's deepening role in the war against Islamic State militants, as Prime Minister Stephen Harper appears ready to seek parliamentary approval to once again send the country into a Middle East conflict.

In interviews Sunday, NDP foreign affairs critic Paul Dewar said his party opposes a combat role for Canada in the campaign against the Islamic State (IS), while Liberal critic Marc Garneau said the government must be crystal-clear on the objectives and role for Canada's military to guard against an open-ended commitment and mission creep.

Both MPs slammed the government's lack of transparency on the issue so far – including Mr. Harper announcing while in New York City last week that the United States has asked for Canadian assistance in the fight as Washington seeks to build as broad a coalition as possible.

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Mr. Harper said cabinet would discuss this week a request from the U.S. to support air strikes against IS forces. The most likely combat contribution would be a half-dozen CF-18 fighter jets and tanker aircraft that conduct in-air refuelling for war planes, in addition to extending the current non-combat effort of providing training for Kurdish fighters in northern Iraq and providing airlift capability.

There are some questions about whether Canadian Forces can contribute much in the way of combat support. Budget cuts at the Department of Defence have curtailed maintenance work on the CF-18s and it could take some time to deploy the jets or have them fly a lot of sorties, said David Perry, an analyst at the Canadian Defence Associations Institute.

"Over the last three years, the big focus on the reductions to the Department of National Defence has been on its operating budget, so it has less flexibility now than it had historically to undertake a mission and to do it without getting extra funding," he said.

A spokeswoman for defence Minister Rob Nicholson, Johanna Quinney, said the Royal Canadian Air Force "has the capacity to meet its operational needs."

Both the Prime Minister and Foreign Minister John Baird have vowed that any combat deployment would be put to a vote in the House of Commons, where Conservatives hold the majority of seats. Mr. Harper did not disclose the exact nature of the assistance sought, and it is possible the coalition – which now includes France and Britain – may not need Canada's CF-18s, but would instead seek a support capability and an extended training mission for special forces now operating in the Kurdish northern Iraq.

Mr. Dewar, who travelled to the country this summer with Mr. Baird, said his party would not support a combat role for Canada but wants to see a much greater humanitarian operation as Iraq struggles to cope with more than one million internally displaced refugees.

"With everything we've seen so far, it's hard to see how we can support the government," the NDP MP said. He said Iraqis, including Kurdish leaders, requested support for the humanitarian mission. "Based on what is actually needed and what I think Canadians would support, we would be the first ones up for the support for the humanitarian assistance needed to save lives. But we couldn't get behind the kind of ill-defined combat mission these guys are talking about so far."

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While the Conservatives can expect NDP opposition, they are counting on the Liberals to give bipartisan support for a combat mission.

Mr. Garneau was non-commital. Appearing on CTV's Question Period on Sunday morning, he appeared to suggest the Liberals would not support a motion that Canada join the air campaign. But in an interview after the show aired, he said his party wants to see the government's motion before deciding whether to support it, including whether the option would authorize air strikes in Syria as well as Iraq.

The U.S. is hitting IS targets in Syria but many of its coalition partners are reluctant to get involved in that country, where the militant group was part of a broader rebellion against Syrian dictator Bashar al-Assad.

"Let's see what the government actually proposes and then we'll look at it and make a decision," Mr. Garneau said. "We're not going to vote for something that is not clearly put in front of us. … Are they going to be talking about deployment just in Iraq or deployment beyond Iraq, and what are the other parameters surrounding it?"