Prime Minister Justin Trudeau says the record-breaking shot by a Canadian special forces sniper is something that should be "celebrated."

It is also well within the realm of the military's advise and assist role in Iraq, he said.

During a marathon news conference Tuesday marking the end of the parliamentary sitting, Trudeau rejected recent criticism by NDP Leader Tom Mulcair, who said the incident draws into question the Liberal government's claim that Canada is involved in a "non-combat" mission against ISIS.

"What happened there is something to be celebrated for the excellence of the Canadian Forces, and their training and the performance of their duties," the prime minister said Tuesday.

The shot that killed an ISIS extremist was taken by the member of the military's elite counterterrorism unit, Joint Task Force 2, sometime last month.

A Canadian 3RCR Battlegroup sniper walks up a hill to his position during a long-range reconnaissance mission near Kabul, Afghanistan in 2003. Canadians have long demonstrated proficiency as snipers. (Stephen Thorne/Canadian Press)

At 3,540 metres, it is the longest confirmed kill shot in military history.

Defensive combat

National Defence has refused to discuss precise details of the operation, but said the shot helped fend off an impending attack on Iraqi security forces.

Trudeau said it was "entirely consistent" with Canadian government policy governing the mission, and on a more basic level, it's something he believes "Canadians expect our Forces to be doing" when allies are under threat.

"The advise and assist mission that our Canadian Forces are engaged in in northern Iraq has always had an element of defence of, obviously, Canadian troops and of our coalition partners," Trudeau said. "That is something that is integral to this mission. And that is something that has always been followed."

But Mulcair countered on Tuesday that while Trudeau was in opposition he described what's been going on in Iraq as combat, but now he doesn't.

"It's clear, when you're shooting at someone and killing them on the front line, you're in a combat mission," Mulcair said.

Former diplomat Ferry de Kerckhove said he believes the Canadian public understands that being in harm's way does not necessarily mean you are engaged in "offensive combat." He said the more than two-year-old debate is something that is politically manufactured.

"It upsets me that this achievement, the saving of Iraqi soldiers and ensuring they don't fall into a trap, is being kind of demonized through this 'combat' versus 'non-combat' game. And I really think it's a game," said de Kerckhove, who served as Canada's ambassador to Egypt, Indonesia and as high commissioner to Pakistan.

"It's a dangerous job and we should honour what the guy achieved."

The killing an ISIS fighter is not something most Canadians would take issue with, given the "horror of their behaviour," he said.

Trudeau responds to Canadian Sniper's record shot 1:10

Canada has roughly 200 special forces troops in Iraq, where they have been helping plan and facilitate Kurdish forces who are attempting to retake the country's second-largest city, Mosul, from ISIS control. The elite soldiers are supported by a Role 2 combat hospital, a detachment of CH-146 Griffon helicopters, a C-140 surveillance plane and an air-to-air refuelling aircraft.

Both the Liberals and the Conservatives before them have described the operations of special forces in northern Iraq, where they have been assisting the Kurds since the fall of 2014, as a "non-combat role."

The term, however, has proved to be somewhat elastic.

Government ministers and military leaders have been called upon to explain how firefights, guiding in airstrikes against ISIS positions and even, in 2015, the friendly fire death of one Canadian soldier cannot be deemed "combat."

Shoot to protect

The debate seemed to have been put to rest last fall, when military officials acknowledged Canada's highly trained soldiers routinely open fire on ISIS fighters in order to protect civilians and their Kurdish allies.

Last week, Mulcair wrote to Trudeau, saying the sniper incident "seriously calls into question your government's claim that Canadian Forces are not involved in direct combat in Iraq."

He insisted the House of Commons be allowed to debate the "change" in the mission, which is set to expire on Friday.

Defence Minister Harjit Sajjan has signalled since early spring that Canada would remain in Iraq while the battle for Mosul was underway. Iraqi forces are still locked in a deadly house-by-house clearing of extremists from the neighbourhood known as Old Mosul.

A CH-146 Griffon lands in a field northwest of Erbil. The helicopter detachment is a recent addition to Canada's mission in Iraq, which officially is one to advise and assist allied forces in Iraq. (Murray Brewster/CBC)

U.S. coalition commanders told CNN on Monday they believed "several hundred" ISIS fighters remained in the shrinking pocket along with as many as 100,000 civilians.

It has taken months to capture the city, and coalition commanders estimate it could more time still as Iraqi and Kurdish troops take care to ensure innocent lives are spared.