President Barack Obama has committed the United States and its allies to an open-ended military campaign to "degrade and ultimately destroy" the Islamic State of Iraq and the Levant terrorist group in Iraq and Syria.

US bombers in Iraq will no longer be operating under orders to defend American personnel and engage in humanitarian strikes, but will lead an international effort to seek out and destroy ISIL targets where ever they can be found, while opposition forces in Syria will be trained and armed by an international coalition.

The President has ordered a further 475 US troops to Iraq to continue advising and assisting Iraqi forces, bringing the total number of US troops in Iraq to around 1600. The President emphasised again that no US troops would be involved in combat on the ground against ISIL.

Speaking from the White House on the eve of the 13th anniversary of the September 11, 2001 attacks the President sought to reassure Americans that the nation was not being dragged into a new war, while demonstrating a strength and resolve his critics say he has so far lacked in combating ISIL.

Mr Obama described IS terrorists as "unique in their brutality."

"They execute captured prisoners. They kill children. They enslave, rape, and force women into marriage. They threatened a religious minority with genocide. In acts of barbarism, they took the lives of two American journalists – Jim Foley and Steven Sotloff," he said.

He said it was "core principle" of his presidency that "if you threaten America, you will find no safe haven."

He said that now that Iraq had formed a new and more inclusive government the US was in a position to step up its efforts to destroy ISIL. He laid out a four point strategy that would see a systematic campaign of airstrikes against ISIL fighters in Afghanistan as well as in Syria, an increase of support and training for local forces fighting IS, including the deployment of an extra 475 US troops to assist.

"As I have said before, these American forces will not have a combat mission – we will not get dragged into another ground war in Iraq. But they are needed to support Iraqi and Kurdish forces with training, intelligence and equipment," he said. He said the US would not cooperate with the Assad regime that is fighting ISIL in its ongoing civil war, and called on Congress to authorise additional resources and authority to train select rebel forces in Syria.

Thirdly the US would seek to cut off funding and the flow of foreign fighters to ISIL and use its counterterrorism capabilities to prevent attacks from the group.

Finally he said the US would continue to provide humanitarian assistances to civilians displaced by ISIL. "This includes Sunni and Shia Muslims who are at grave risk, as well as tens of thousands of Christians and other religious minorities. We cannot allow these communities to be driven from their ancient homelands.

In keeping with the doctrine for the use of US force outlined earlier this year, Mr Obama said the campaign would be an international effort led by the United States rather than a solely American operation.

"America will be joined by a broad coalition of partners. Already, allies are flying planes with us over Iraq; sending arms and assistance to Iraqi Security Forces and the Syrian opposition; sharing intelligence; and providing billions of dollars in humanitarian aid," he said.

"This strategy of taking out terrorists who threaten us, while supporting partners on the front lines, is one that we have successfully pursued in Yemen and Somalia for years," he said.

He did not specify how the the international community would contribute to the campaign, but speaking earlier in the evening a senior White House official noted that Australia had already contributed to air supply efforts in the region.

He said it had not yet been decided how each nation would contribute to what he called "the kinetic" campaign against ISIL – that is, the bombing campaign. But said Saudi Arabia would be a key partner in the strategy and would host the training of Syrian rebel forces, while Sunni regional governments and tribal leaders had already offered their support for the campaign.

Speaking ahead of Mr Obama's address, Prime Minister Tony Abbott said no "specific request" for further military assistance from Australia "has yet been made".

"A specific request for military assistance in the form of air capability, in the form of military advisers, could come, it could come but it hasn't yet come and if it does come it will be dealt with in the normal way," he said.

The President did not give a timeframe for the operations, though the White House official said it could take some time for targets in Syria to be identified. He said special forces troops would not be needed on the ground for targeting and US troops would not go into Syria.

Asked what victory would look like, the official said the ISIL territory would be shrunk, its ranks decimated and its threat diminished. He was not able to say how long this might take.

The Republican Speaker of the House of Representatives, John Boehner he supported plans to equip and train Syrian opposition forces and Iraqi troops, but said this could take years.

"The president tonight recanted his earlier dismissals of ISIL's capabilities and rightly acknowledged the grave and growing threat posed by the spreading global epidemic of radicalised Islam," he said in a statement. "He has finally begun to make the case the nation has needed him to make for quite some time: that destroying this terrorist threat requires decisive action and must be the highest priority for the United States and other nations of the free world."

Having been elected on a promise to end America's wars Mr Obama has appeared reluctant to use US military might against ISIL in the past and has been criticised for playing down its threat.

Even as Republican hawks in Congress criticised him, Mr Obama's position was in keeping with a public that polls revealed did not want to become involved in another war.

That changed with the beheading of the two American journalists. An NBC/Wall Street Journal poll published on Wednesday found that 61 per cent of Americans support military action against ISIL, compared with 21 per cent who supported action against Syria after its use of chemical weapons was revealed.