Defence Minister Kevin Andrews has likened the border between Iraq and Syria to that between New South Wales and the ACT, and says Australia's military operations against Islamic State in both countries could last until 2018.

Yesterday, the Federal Government confirmed it was expanding RAAF operations from Iraq into Syria as part of US-led air strikes, with the first bombs expected to be dropped within days.

The Minister has ruled out the prospect of Australia deploying ground troops to support the airstrike commitment.

"We're in Iraq at the invitation of the government of Iraq — we're defending Iraq," Mr Andrews said.

"It makes sense to be able to go over the border because it's a bit like the border between New South Wales and the ACT."

Mr Andrews also said it was difficult to say how long Australia would remain entrenched in the Middle Eastern conflict.

"Two, three years. I can't say for exact, exact terms," Mr Andrews told Channel Nine this morning.

But, speaking from Papua New Guinea, Prime Minister Tony Abbott was not prepared to make any predictions about the length of Australia's commitment.

"We don't really want to put a particular timeline on this other than to say that they'll be there as long as needed but no longer than necessary," he said.

"With all of our military commitments we make them because there is a job to be done.

"Our armed forces personnel do the job effectively and professionally and when the job is done they come home."

Mr Abbott said the Government's commitment to military action, combined with the announcement of a special 12,000-place additional refugee intake, illustrated "a big-hearted, generous response".

"We are country with a good heart, with a big heart as well as a strong arm," he said.

The Federal Government has received conditional support from Labor to start bombing in Syria.

The military action will be subject to an annual review process.

Labor backbencher Stephen Jones said the Government had been "shamed" into making the one-off additional humanitarian commitment.

He has acknowledged the need for military action in tandem with the refugee intake.

"I'm not comfortable with Australian forces being in a war zone for one day longer than they need to," he said.

"But we know that we cannot sit on our hands when it comes to [IS], we need to be part of an international effort to bring these terrorists to book."

He said the military's aims were to "knock out" terrorist facilities, training bases and control and command bases.

Bishop discusses expanded role with US secretary of state

Foreign Minister Julie Bishop has confirmed she had a "long conversation" with US Secretary of State John Kerry overnight to discuss Australia's expanded role.

"There must be a humanitarian response and we're part of that. There must be a security response, we are also part of that," she said.

The military aims to "knock out" terrorist facilities and training bases in Syria. ( Defence Force )

"There must be a political solution and that was the topic of discussion with Secretary Kerry last night — what we can do to ensure that there is a regime in Syria that doesn't attack its own people and that we can defeat a terrorist organisation that's carrying out the most brutal and violent attacks on people we've seen."

Ms Bishop said she had also spoken to her counterparts in the United Arab Emirates and Saudi Arabia and expected more conversations with European and Middle Eastern representatives in the coming days about the additional military and humanitarian measures they would be prepared to take.

She also noted continued talks about the role Russia is playing in the conflict.

"Of course we're concerned about Russia's support for Assad's regime — that is nothing new. Russia has been supporting the Assad regime for some time," she said.

"Our focus is on the civilians, our focus is on ensuring that Australian foreign terrorist fighters do not take up arms with this terrorist organisation and our focus is ensuring that they don't come back to Australia and carry out terrorist attacks in our country."

Questions on cost of extending airstrikes to Syria

Treasurer Joe Hockey said he was not expecting the extended bombing operation to have a big impact on the cost of Australia's military action in the Middle East.

"We're informed that there won't necessarily be additional sorties and we don't at this stage envisage dramatically increasing our presence in the region," he said.

"As for current expectations about cost and military action, they're not much different at all to our current costs."

Mr Hockey said the final cost would depend on factors like the number of ordinances dropped, but it is expected the broader operation will include the existing planes and similar sorties.

This year’s budget papers noted total spending on "Operation Okra" in Iraq for this financial year is estimated at $390.8 million, along with $418.3 million projected over the forward estimates.

The Government has also predicted the cost of resettling 12,000 additional refugees will be close to $700 million.

"The costs depend ultimately on the speed at which we can process applications," Mr Hockey said.

"There are some estimates that we can do it in a two-year period and some estimates that it may take a little longer."