Operation Inherent Resolve spokesman Colonel Steve Warren also said that troops now being trained by Australians and New Zealand forces in Iraq would be crucial in driving the militant group out of its Iraqi stronghold of Mosul, which he said could be the group's "last stand" in the country.

Speaking to Australian reporters, Colonel Warren said the US was still looking to all countries to contribute more to the US-led campaign. He acknowledged Australia's disproportionately large contribution, though he added that "enough is never enough".

Asked specifically about Mr Turnbull's call at the weekend for Europe and NATO countries to "step up", Colonel Warren said: "He's right in the fact that other nations who are part of this coalition, there's a lot more room for them to grow than for the Australians and the New Zealanders.

"On the list of people who need to step up, the Australians are at the very bottom of that. They've stepped up already. But it's time to see others."

Australia has about 300 forces at Camp Taji, north of Baghdad, who are training the Iraqi army, with help from about 100 Kiwis. It also has about 80 commandos helping direct Iraqi military operations, six Hornet fighter jets, air refuelling and radar planes and dozens of staff embedded with the US in headquarters across the Middle East.

Colonel Warren described Mosul as the Islamic State's "centre of gravity" in Iraq and said driving it out of the city - Iraq's second-largest - would be the "keystone" to the group's collapse in that country.

"We believe they will fight tooth and nail."

Leaving Mosul would be a "risky and deadly move for them" given there is very little in the way of population centres between that city and the Syrian border, leaving them exposed.

"An assault on Mosul ... potentially could turn out to be something akin to a last stand for them, at least here in Iraq."

But he offered a relatively cautious assessment of how quickly Mosul can be recaptured, saying only that it was "possible" the Iraqi military offensive would happen this year. This is less definitive than Iraqi Prime Minister Haider al-Abadi's forecast that the militants will be driven out by the end of 2016.

"They have to get a few more brigades through training, a few of the already trained brigades back through training. They've been fighting hard so they're going to require a bit of refit time as well. It's too soon to put a time frame on it."

Australian soldiers are training Iraqis, including many from Mosul, in skills ranging from marksmanship to combat tactics and leadership.

Colonel Warren said these forces would be crucial. The recent success of the Iraqis in largely retaking the city of Ramadi would be a "template" for the Mosul fight.

However, the Ramadi campaign relied heavily on Iraqi special forces, known as the counter-terrorism service or CTS. Mosul by contrast will be too big a battle for those elite troops, meaning it was "one hundred per cent certain" that regular Iraqi army forces will need to shoulder much more of the combat burden," Colonel Warren said.

"Mosul is too large and too complex a fight for only the CTS. Combat power will have to come from the regular army. So they will play a key and critical role in liberating Mosul.

"There's no question about it."

Colonel Warren said he still believed the fight against the Islamic State would take "years".

"But whether it's one [or] three, it's hard to say ... They are in a defensive crouch. They are now reacting to us, as opposed to us reacting to them and that really is the litmus test ... The end is inevitable but they're still a dangerous foe at this point."

He said that the Mosul campaign will "absolutely" involve more so-called "shaping operations", such as air strikes, to confine and weaken Islamic State forces.

"We expect the Australian contribution we've seen so far will be a major factor in operations to retake Mosul."

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