Counter-insurgency expert and strategist David Kilcullen says the fight against Islamic State (IS) is a long way from being won and says there is a strong case for Australia to join military air strikes in Syria.

Prime Minister Tony Abbott strongly hinted Australia would join the United States in carrying out air strikes against IS in Syria after receiving a formal request from the president to extend its strikes in Iraq to cover Syria.

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Dr Kilcullen said the volume of Australian air craft would not be a game-changer to air strikes in Syria, but that joining the US would add weight and professional capability.

"I think you could also make the case that ISIS is now really operating a single entity that crosses the boundaries of Iraq and Syria," he said.

"And you could completely defeat ISIS in Iraq ... but if you don't then do something about its safe haven in Syria and its ability to move at its will between Syria and Iraq, it'll just reinvigorate itself in a matter of weeks to months.

"So for the ultimate, long-term objective of defeating ISIS, I think it is utterly essential to take the fight to them in Syria. Now whether that's something that Australia wants to be part of is a question for the Australian Government.

"But I think there's a military argument to be made that doing things in Iraq without also making an attempt in Syria is a bit of a waste of time."

Dr Kilcullen said it was also important for Australia to define the extent of its commitment.

"There are still significant ISIS operations going on in Libya and in the Sinai Peninsula. There's even a significant ISIS activity going on in Afghanistan," he said.

"So where do you draw the line? I think it is important to decide what constitutes the extent of our commitment.

"Are we playing, as the Americans say, zone defence or are we playing the man? Are we following ISIS or are we declaring some kind of territorial limit to the operation?

"But look, I think there's a solid military argument for doing the job in Syria as an adjunct to doing the job in Iraq."

'Nowhere close to beginning to roll ISIS back'

Dr Kilcullen said the Syrian government had lost substantial amounts of ground to ISIS and the fight was far from won.

David Kilcullen said there was a strong case for Australia to extend its strikes in Iraq to cover Syria. ( Australian Defence Force: Sgt Pete )

"ISIS now controls more than half of Syrian territory," he said.

"In Iraq, there's been a huge political crisis over the summer between the former prime minister, Nouri al-Malaki, and current the prime minister [Wael Nader al-Halqi], and there's a whole series of protests going on across Iraq.

"So what we're seeing is the lack of the political consensus and unity that would be needed to mount an effective military campaign so the chance of seeing Mosul being recaptured any time soon I think is very slim.

"And the chance even of Ramadi, the capital of Anbar that was captured by ISIS back in May, being retaken is also pretty slim.

"So right now unfortunately, we're more in a containment mode of trying to stop ISIS from continuing to expand, nowhere close to beginning to roll ISIS back."