Australian special forces have been cleared to start work on the ground in Iraq, helping local troops as they face the grinding task of driving Islamic State fighters out of their stronghold towns and cities.

Prime Minister Tony Abbott is expected to announce on Tuesday that the final legal hurdles with the Iraqi government have been cleared, meaning the Australian commandos can begin their "advise and assist" work with the Iraqis.

The paperwork from Baghdad came as RAAF Super Hornet fighters returned safely to the United Arab Emirates from their first combat mission in Iraq, providing air cover for local troops in the country's north, though they did not launch weapons.

But even as the RAAF missions began, Defence Minister David Johnston acknowledged that the Islamic State fighters were quickly adapting to air strikes.

"I think that's pretty certain that they will adapt very quickly not to be out in the open where the Iraqi security forces can call in an air strike," he said.

Air strikes would less frequently be against fixed targets but would rather be in support of Iraqi and Kurdish forces on the ground, as the US was currently doing in the besieged northern Syrian town of Kobani, he said.

But Senator Johnston acknowledged this would be harder in the major Islamic State strongholds in Iraq such as Fallujah, Ramadi and Tikrit, where the Iraqi forces would need to "step up" and drive the militants out.

"I think it was always going to be … that the Iraqi security forces would have to step up and go into these towns and clean them out."

The former Chief of Air Force, retired Air Marshal Geoff Shepherd, said the militants would "melt among the population".

"At the start of their campaigns, they were massing as a military force … but they're smart enough now to melt back into the population and it will be more difficult to contain them by air power alone."

He warned that this could raise the risk of civilian casualties, which would need to be avoided.

"We're not going to win this just by bombing. We're going to win it by challenging their ideology and their view of the world, and every time we kill an innocent civilian … that drives people into the arms of jihadis."

Syrian Kurds warned that air strikes were failing to halt the advance of the Islamic State in that country's north as they lay siege to a key town near the border with Turkey.

"Air strikes alone are really not enough to defeat ISIS in Kobani," said Idris Nassan, a senior spokesman for the Kurdish fighters trying to defend the town, using an alternative name for the militants.

"Each time a jet approaches, they leave their open positions, they scatter and hide. What we really need is ground support. We need heavy weapons and ammunition in order to fend them off and defeat them."

Up to 200 Australian special forces soldiers will work in Iraq, bolstering the local troops on the ground. It is understood that some left Al Minhad air base in the United Arab Emirates before the final legal clearance, to begin work.

Their focus will be on strengthening the leadership ranks among the Iraqis to avoid the military collapses that allowed the Islamic State to seize large swathes of territory in recent months.

But Fairfax Media understands that among the troops will also be elite joint terminal attack controllers – special forces experts who can call in air strikes from the ground with great accuracy.

They would work with the RAAF Super Hornets and Wedgetail surveillance and control planes to hit moving Islamic State targets while the Iraqi army fight them on the ground.

The Australian government needed a detailed legal agreement with the new government in Baghdad that ensures Australian Defence Force personnel have the necessary legal cover if, for instance, they are involved in the deaths of civilians.