Australia's elite special forces troops in Iraq will be gathering intelligence and targeting militants, either through ground operations or by directing air strikes, according to a senior defence force insider familiar with the SAS role.

The claim runs counter to the impression being given by Prime Minister Tony Abbott, who continued to say on Wednesday that Australian forces would be "military advisers" to Iraqi and Peshmerga fighters and would "not themselves normally engage in actual combat".

It came as the US top military commander, General Martin Dempsey, opened the door to a deeper combat role on the ground for coalition forces, and former chief of army Peter Leahy said the Abbott government should be prepared to consider expanding ground operations if military commanders said it was needed.

The now-retired senior defence insider, who spoke on the condition of anonymity, said it was an accepted fact within the Australian Defence Force that special forces troops were being deployed because they are the ADF's most lethal force.

"You don't send in the SAS to run seminars and give white-board presentations back at headquarters," the former top tactician said.

"These guys are our most highly trained killers, and that's what they will be doing."

He said the claim that they will be used purely as "advisers" to the Iraqi security forces and the Kurdish Peshmerga was "pretty absurd".

Mr Abbott has been pressed repeatedly on this subject since declaring on the weekend that Australia would send 600 personnel to the region, with as many as a third of those drawn from the SAS Regiment based at Swanbourne in suburban Perth.

He said: "They'll be there to act as military advisers," and while entitled to respond if attacked would generally work out of battalion headquarters.

"Obviously they'll be moving around with those unit headquarters, but the point I stress is that there is no intention for Australia to conduct independent combat operations inside Iraq."

The former defence force officer said the term "battalion headquarters" implied it was far removed from the battle but in truth it would be a "somewhere for the SAS commandos to get a secure night's sleep", which was very much "in the field".

Mr Abbott's attempts to contain the Iraq mission and assure Australians that its goals are both narrow and achievable, was complicated overnight by remarks by General Dempsey, Chairman of the Joint Chiefs of Staff, who said ground forces could be recommended if the war goes badly.

Professor Leahy, the former army chief who now heads the University of Canberra's National Security Institute, said the government should be ready to expand the ground role if it proved necessary.

"The situation may develop as the campaign goes on, and you would be ill advised to think that the plan you go in with survives first contact with the enemy," he said.

He stressed any decision to escalate could only be recommended by military commanders but ultimately had to be taken by politicians.

"That means public engagement, public information, discussion and debate," he said.

Officially, Labor remains aligned to the government's approach but privately, some opposition MPs are critical of the insistence on playing down the combat role, with one telling Fairfax Media it was puzzling.

"Why not just come out and say that we are there to apply our lethal military power? Why hide behind words like 'advising' and not putting 'boots on the ground'?"

With so many details yet to be worked out, including the precise nature of Australia's military contribution, Iraq's ambassador to Australia, Mouayed Saleh, declined to explain his government's interpretation of rules governing SAS "advisers".

"We don't want to put it literally as 'confined-to-headquarters or to-the-field'," he told Sky News.

"But whatever the advice dictates at that moment, whatever's needed, then that's what happens."

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