“I want to reassure the Australian people that it will be as long as it needs to be, but as short as it possibly can be,” he said.

Image Prime Minister Tony Abbott, right, with the chief of the Defense Force, Air Chief Marshal Mark Binskin, during a news conference in Canberra on Friday. Credit... Alan Porritt/European Pressphoto Agency

Mr. Abbott spoke after Australia’s National Security Committee and its full cabinet met early Friday. Mr. Abbott said Australia would join the airstrikes at the request of the Iraqi government but was still awaiting final legal agreements with Iraq covering the deployment of special operations personnel on the ground. Those agreements were likely to be finalized within 24 hours, he said.

Mr. Abbott has been vocal about what he says is the need to fight the Islamic State, also known as ISIS, in the Middle East and in Australia, where he says the militant group has ordered followers to commit murders in public. Intelligence officials here say that about 70 Australians have joined the group in the Middle East.

On Wednesday, the country’s Parliament passed legislation urged by Mr. Abbott that would expand telecommunications surveillance and the police’s power to detain suspects, as well as subject journalists to possible prison terms for unauthorized reporting on intelligence matters.

Bill Shorten, leader of the opposition Labor Party, supported the deployment announced Friday, calling it “a sensible decision in a most difficult set of circumstances.” But a senator with the Greens party, Christine Milne, criticized what she called Mr. Abbott’s “rush into another U.S.-led, multiyear war in Iraq.”