Six Australian F/A-18F Super Hornet jet fighters will launch airstrikes against Islamic State targets in northern Iraq within days as part of the U.S.-led coalition, officials said on Friday.

The announcement of an Australian combat role on Friday has been widely anticipated since the Super Hornets were pre-deployed to the United Arab Emirates more than two weeks ago in response to a formal request from the United States for specific contributions to the international coalition.

Prime Minister Tony Abbott told reporters that the deployment to Iraq "could be quite lengthy. Certainly, months rather than weeks."

"Yes, it is a combat deployment, but it is an essentially humanitarian mission to protect the people of Iraq and ultimately the people of Australia from the murderous rage of the ISIL death cult," Abbott said.

"ISIL must be disrupted and degraded at home and abroad, so it is absolutely in Australia's national interests that this mission go ahead," he said.

Still, he said total victory over Islamic State would be difficult to achieve.

"If we could degrade them to the point where they no longer existed, that would be obviously the best possible result," he said, adding, "It is very difficult to eliminate an idea."

Defense Force Chief Air Chief Marshal Mark Binskin said the airstrikes would start "over the coming days," but would not be more specific.

The seven Cabinet ministers who make up the government's National Security Committee approved the deployment after an official request was received from Iraq overnight.

Two unarmed Australian air force planes -- an E-7A Wedgetail surveillance and communications jet and a KC-30A refueling plane -- joined operations over Iraq from the al-Minhad Air Base outside Dubai for the first time on Wednesday in support roles.

The government says the number of Super Hornets could soon be increased to eight.

The Australian deployment also includes a 200-strong ground force, including special forces, to advise security forces inside Iraq, plus 400 air force personnel.

The special forces will also deploy to Iraq to "advise and assist Iraqi security forces" once the appropriate legal arrangements were in place with the Iraqi government, Abbott said.

Abbott has restricted combat operations to Iraq and has ruled out Australian troops fighting on the ground.

Australia is among dozens of countries from Europe, Middle East and including Canada that have signed up to the U.S.-led coalition fighting the Islamic State in Iraq and Syria. Contributions vary and include military assistance and humanitarian aid as well as carrying out airstrikes.