Prime Minister Tony Abbott has strongly hinted Australia will join the United States in carrying out air strikes against Islamic State (IS) in Syria.

Mr Abbott confirmed Australia had received a formal request from the US to extend its strikes in Iraq to cover Syria.

"We're not going to rush a decision, but we will make a decision in the next couple of weeks," he said.

He acknowledged legal differences between carrying out strikes in Iraq compared to Syria, but indicated that would not be a roadblock.

Key points: US requests Australia expand role in Syria

US requests Australia expand role in Syria Abbott strongly hints Australia will agree to request

Abbott strongly hints Australia will agree to request Labor will seek explanation of legal basis

Labor will seek explanation of legal basis Australia already involved in air strikes in Iraq

"While there is a little difference between the legalities of air strikes on either side of the border, there's no difference in the morality," he said.

"Whether it's operating in Iraq or Syria it is an absolutely evil movement and in the end, when they don't respect the border, the question is why should we?"

The Prime Minister phoned Opposition Leader Bill Shorten this morning to tell him he had received a formal request from the Americans.

Mr Shorten said he wanted to know the legal basis for any expansion of Australia's role in Syria.

"We will be seeking an explanation of a legal basis upon the proposition which the US has asked us; is it legal what the US has asked us to do?" Mr Shorten said.

Australia spent weeks negotiating an agreement with the Iraqi government to join Coalition forces there.

Cabinet Minister Christopher Pyne said this morning the Government "would do everything on a very strong legal basis" if it were to take further action in Syria.

But Professor of Law at Australian National University Don Rothwell warned there was no clear legal basis for military intervention from Australia in Syria.

"There is no Security Council resolution, Australia can't exercise a right of self defence... and certainly the fact that the US has made an invitation is not sufficient," Professor Rothwell told ABC24's Capital Hill program.

"I think the only way that Australia could perhaps proceed would be to actually radically adjust Australian doctrine on these matters, to take a more expansive view of self defence, to take a more expansive view in terms of following terrorists into a state which is harbouring terrorists."

But Professor Rothwell warned that might cause concern in the region to have Australia taking such an approach.

Australians being stopped at airports almost every day: Abbott

Mr Abbott pointed out the US has been carrying out air strikes in Syria for many months now and Canada and Britain have indicated they are prepared to join them.

Mr Abbott said Royal Australian Air Force planes were already involved in refuelling Coalition planes for strikes in Syria and controlling air space for strikes across the border.

He revealed "impressionable" young Australians were being stopped at airports "almost every single day" seeking to travel to the Middle East to join IS.

It followed confirmation yesterday that seven young Australians were intercepted at Sydney Airport in the past week.

Mr Abbott also warned the consolidation of a terrorist state in eastern Syria and northern Iraq would be "a catastrophe for the world" and "a disaster for Australia".

"It's very important Australia play its part in the campaign to disrupt, degrade and ultimately destroy this death cult."

RAAF warplanes have been bombing IS targets in Iraq since September, but Liberal MP and chairman of Federal Parliament's intelligence and security committee Dan Tehan said it was time for Australia to step up its commitment.

"We have the world's greatest humanitarian crisis occurring at the moment in Syria with over 9 million people displaced," he said.

"They're raping, they're murdering, they're pillaging and we need to stop the foreign fighter flow to the caliphate in Syria."

But earlier this week, Defence's Joint Chief of Operations David Johnston played down the effect that Australian involvement in Syria would have.

"The contribution of Australia, while always welcome, isn't a game changer one way or the other, in part there's a zero-sum gain," Vice Admiral Johnston said.

"Whether we're operating in Iraq or Syria, the capacity is the same."

Vice Admiral Johnston also said bombing Syria would increase the level of risk involved in Australia's mission.

"I don't underestimate the complexity of our commencing operations in Syria, it is a significantly more difficult environment," he said.