The former commander of coalition forces in Iraq and Afghanistan, David Petraeus, is encouraging Australia to join the coalition air strikes against Islamic State in Syria.

The Federal Government is currently considering a request to expand its support for the US bombing campaign.

Mr Petraeus, a retired four-star general and former CIA director, made the remarks as he delivered the Lowy Institute lecture in Sydney on Wednesday night.

"Taking such an action, together with the other coalition members, will do damage to ISIS," he said.

"It will add, it will complement, augment the actions of the US and the other coalition partners that are indeed engaging ISIS not just in Iraq but in Syria as well."

Mr Petraeus admitted the act may help the cause of Syrian president Bashar al-Assad.

"To be sure, ISIS is an enemy of Bashar al-Assad and also of the moderate opposition that we have been trying to support and that we have to support ultimately to defeat ISIS and, by the way, Jabhat al-Nusra, the Al Qaeda affiliate, the Khorasan group established to try to project terrorism into Europe and the United States," he said.

"So again, you have a situation where going after one could actually help another that is indeed a despicable, barbaric autocratic regime responsible for the deaths of somewhere around 250,000 Syrian citizens."

Ultimately though, Mr Petraeus encouraged Australia to consider going ahead with the airstrikes.

"At the end of the day there's a reason why the coalition countries and the US have taken that action and again, I think that that is one that ultimately would be one that Australia should consider with that in mind," he said.

Coalition should enlist moderate Al Nusra Front members

Mr Petraeus made global headlines this week for suggesting that the US should co-opt moderate members of Al Qaeda's Al Nusra Front, in its fight against Islamic State in Syria.

In his speech on Wednesday night, he said the Coalition had already used similar tactics in Iraq.

"Well, it should not be cooperating with the leaders of an Al Qaeda affiliate, Jabhat Al Nusra.

"It should certainly consider doing what we did in Iraq where we were confronted by an industrial strength extremist insurgent movement, Al Qaeda in Iraq and associated Sunni Arabic insurgents and we recognise that we could not kill or capture our way out of this industrial strength insurgency.

"We needed to strip away some of the fighters, some of the sheiks of the tribes that were either tactilely or actively supporting Al Qaeda."

Mr Petraeus devoted the majority of his address to challenges in the Asia Pacific, and said the US had too frequently been seen to neglect the region, in favour of the Middle East.

"First and foremost, we need to avoid any foreign policy concept that appears to elevate the priority of either the Middle East or the Asia Pacific at the expense of the other," he said.

"Doing so will only serve to make it harder for us to achieve our ends in both of these critical theatres."

Mr Petraeus said the US had vital national interests in both regions, and cannot afford to withdraw from either.