Canadian soldiers are authorized to shoot first in Iraq as soon as they detect "hostile intent" from Islamic State fighters, MPs were told Tuesday as the House of Commons approved the Trudeau government's enlarged and more dangerous ground deployment there.

The Liberal-dominated House of Commons approved the revised fight against Islamic State, with a motion passing by a margin of 178-147.

The government has withdrawn CF-18 fighters from the aerial war against the jihadis and expanded Canada's ground commitment of special-forces soldiers to 220 from 69. It has also committed to giving small arms and ammunition to forces batting Islamic State.

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But the rules of engagement are fuelling more controversy among critics who argue this operation does not sound like the "non-combat" role pledged by Prime Minister Justin Trudeau.

The Liberals, who made a campaign promise to "end Canada's combat mission" in Iraq, are determined to frame the enlarged commitment as something other than combat.

General Jonathan Vance, Chief of the Defence Staff, was asked Tuesday to explain to the Commons defence committee what rules govern Canada's elite troops in Iraq.

The Canadians are providing advise-and-assist leadership to Kurdish peshmerga fighters battling Islamic State forces and this includes spending significant time on the front lines with troops. Soldiers are also authorized to direct air strikes from the ground in aid of the U.S.-led air campaign.

The general told MPs the Canadian mission is "largely defensive" but troops are given licence to take the first shot when necessary.

"The rules of engagement … allow Canadian forces to defend themselves, [to] anticipate their defence so they can engage a hostile act … or an intent before it materializes," Gen. Vance said. "In other words … we can anticipate to protect ourselves."

Responding to Gen. Vance, NDP defence critic Randall Garrison questioned how what soldiers are doing in Iraq could be labelled a "non-combat" operation.

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"I think for most Canadians that does sound a lot like being engaged in combat," said Mr. Garrison, whose riding includes Canadian Forces Base Esquimalt.

A military spokesman declined Tuesday to say how long this shoot-first rule has been in effect. Troops have been in Iraq since the fall of 2014 but rules of engagement are modified from time to time.

Stuart Hendin, a legal scholar in the area of armed conflict and human rights, said the "hostile intent" rule means something as simple as an opposing soldier levelling his rifle and pointing it at a Canadian or their Kurdish allies.

The military says Canadian soldiers are not in combat in Iraq because they are not deliberately partaking in offensive operations.

Mr. Hendin disagrees.

"Whenever you are putting people in harm's way, where there is an armed conflict going on, that is combat," he said.

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"If they are in a front line where they are potentially exposed, that is combat."

A spokesman for the Department of National Defence said soldiers have been in firefights with Islamic State opponents four times since the fall of 2014.

Those voting in the Commons Tuesday included former prime minister Stephen Harper, whose Conservative government first deployed Canadian Armed Forces against Islamic State in 2014.