ABC News Online

US president Barack Obama has vowed to attack Islamic State militants "wherever they exist" in a speech laying out a strategy for expanding airstrikes in Iraq and, for the first time, striking targets in Syria.

Mr Obama stressed in a speech broadcast live to the nation that he will not send US combat troops to fight IS, and that the US will act in concert with a broad coalition including Western allies and Arab states.

IS, which the US refers to as ISIL, has carved out what it calls a "caliphate" from broad areas in Iraq and Syria and uses savage methods that have included the beheading of many prisoners, including two American journalists.

"I have made it clear that we will hunt down terrorists who threaten our country, wherever they are," Mr Obama said.

"That means I will not hesitate to take action against ISIL in Syria, as well as Iraq.

"This is a core principle of my presidency: if you threaten America, you will find no safe haven.

"Our objective is clear: we will degrade and ultimately destroy ISIL through a comprehensive and sustained counter-terrorism strategy."

Mr Obama outlined a four-pronged strategy which includes expanded air strikes and sending another 475 troops to train local forces.

US officials said Mr Obama was opening the door to air strikes against the group in Syria.

Pursuing the Islamist militants inside Syria would complement an expanded military campaign to back government forces in Iraq following the formation of a more inclusive government in Baghdad.

Prime Minister Tony Abbott said the Federal Government had not received a request from the US for further military assistance, but he suggested Australia was likely to expand its involvement.

He said any request for further help would be considered by the National Security Committee of Cabinet.

"A specific request for military assistance in the form of air capability, in the form of military advisers, could come," Mr Abbott told reporters in Tasmania.

Opposition Leader Bill Shorten said he expects the Government will brief Labor if and when a request is received.

"We will continue to treat this issue as not a political issue, but as a matter of national security," he said.

Thomas Adams, a lecturer in American studies from the University of Sydney, said it "seems like the United States is garnering Australian support for this intervention".

"And again I think the lack of an end game here is to me, most problematic," he told ABC News 24.

"There's really no good solution at this point.

"I think as one of the key aspects that the United States has failed to realise in the region, that will very much affect Australia as it goes along with American policy, is the idea that Iraq as an entity has ceased to exist for anyone of any importance.

"That is to say as Sunnis in Iraq don't see a value in the way that Iraq previously existed."

Mr Obama is expected to urge Congress to approve $US500 million to provide more arms and training to rebel groups in Syria, a key element in any campaign of air attacks there.

The rebel groups were formed with US encouragement to try to oust president Bashar al-Assad, but Washington did not provide them with the weapons they needed and they have been eclipsed by Islamist and Al Qaeda-associated movements.

After more than 150 US airstrikes in Iraq in the past month, Iraqi and Kurdish forces have halted the Islamic State advance.

Mr Obama had already signaled his willingness to expand the mission to Syria, where IS has a stronghold.

That is a significant shift for a president who has been reluctant to increase the US military footprint in the region and three years ago pulled out the last combat troops from Iraq.

The president is scheduled to speak in a prime-time slot that raises the profile and stakes for his speech, which comes on the eve of the 13th anniversary of the September 11, 2001, attacks on New York and the Pentagon.

Obama builds on anger fuelled by murder of journalists

With the speech, Mr Obama is trying to build on the support that has grown among the American public for military action, partly fuelled by anger over the beheading of two American journalists in the past month, and also appeal to likely international partners.

Mr Obama spoke to Saudi Arabia's King Abdullah earlier as part of that effort, and secretary of state John Kerry, now visiting Baghdad, will be meeting with leaders across the region in the coming days.

The president told congressional leaders on Tuesday that he did not need additional authorisation to carry out his plan, but the White House is eager to have their support, along with that of the international community.

Although US officials say there is no imminent threat from IS militants against the US, there are strong concerns that individuals from the West who went to fight with the group may return to their home countries and attack civilians.

Mr Obama came close to direct military action a year ago in Syria to support what Washington considers more moderate rebel forces fighting Mr Assad, but he held off because of strong opposition in Congress.

Polls this week show the majority of Americans support action against the militants.

More than 70 per cent of Americans support airstrikes in Iraq and 65 per cent support using them in Syria, a Washington Post-ABC News opinion poll found.

An NBC News-Wall Street Journal poll showed 61 per cent said military action against the group was in the interests of the US.

Mr Obama showed a willingness to intrude militarily into Syrian space with an unsuccessful operation in July to try to rescue American prisoners.

This article was originally published at ABC News Online