President Barack Obama on Wednesday vowed to strike the "Islamic State" in Syria and to expand operations in Iraq, seizing command of an international coalition to destroy the jihadist army.

Obama, who wants history to remember him as the president who ended a decade of foreign wars, launched a major new escalation in the Middle East, also pledging to bolster Iraqi forces and to increase military assistance to Syria's opposition.

In a prime-time televised address, Obama said that IS, which has beheaded two US journalists and seized a swathe of land in Iraq and Syria, was a uniquely brutal group even by the blood-soaked standards of the Middle East.

"Our objective is clear: we will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy," Obama said, steeling his nation for a new foreign war.

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

"That means I will not hesitate to take action against ISIL in Syria, as well as Iraq," he said, using an alternative acronym for the self-declared Islamic State.

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

Seeking to preserve his doctrine of replacing strength-sapping wars with easily bogged down land armies, with limited but lethal arms length air warfare, Obama compared the new conflict to anti-terror operations in Somalia and Yemen.

But he made clear in the somber, 14 minute address, that the new front against terrorism will not replicate the exhausting ground wars of the last decade.

"I want the American people to understand how this effort will be different from the wars in Iraq and Afghanistan," Obama said.

"It will not involve American combat troops fighting on foreign soil."

US operations in Somalia and Yemen have largely used drone strikes and air raids, though US special forces did stage a raid in Somalia last year, which failed to snare its target a prominent Islamic militant.

US officials declined to say when air strikes would start in Syria, or which particular IS targets would be in the crossfire.

"We are not to going telegraph our punches," one senior official said on condition of anonymity.

The official also said that Obama would loosen the previous criteria for attacks on IS forces in Iraq, which were stipulated on the need to safeguard US diplomatic personnel and avert humanitarian tragedies.

"If there is an IS target in Iraq that we need to hit, we will hit it," the official said.

The president also made clear that the burgeoning challenge of IS, which has carved out a haven in Iraq and Syria that has made national borders obsolete, had made him change his mind about arming Syrian opposition forces.

He called on Congress to swiftly authorize an operation to train and equip moderate Syrian fighters, and officials. Saudi King Abdullah, to whom Obama spoke earlier on Wednesday, had promised to support the effort, officials said.

"We must strengthen the opposition as the best counterweight," Obama said, warning that Washington would never work with Syrian President Bashar al-Assad to combat IS, arguing he had lost legitimacy by waging a brutal civil war against his own people.

Obama also urged lawmakers to swiftly approve a $500 million request for funding for the Syrian training mission which he first made in June.

He said he believed he had the power to conduct the campaign against IS under existing anti-terror authorizations.

Politically, Obama's address, on the poignant eve of the September 11 attacks in 2001, which pitched the United States into an unresolved confrontation with radical Islam, was an attempt to shore up his position after a few tricky weeks.

Obama's admission last month that he didn't yet have a strategy to take on Islamic State in Syria exposed him to withering political attacks from his Republican foes.

In this speech, in which he promised a "steady, relentless" combination of US air power and the work of foreign partners like the Iraqi government and the Kurds, he repeatedly used the word "strategy."

Speaking from the ceremonial State floor of the White House, looking directly into the camera, Obama said that his new strategy had only been possible because Iraqi politicians had converged and formed an "inclusive"  government to unite the country and take on IS.

He announced the dispatch of another 475 military personnel to help train Iraqi forces to take on the Islamic State.

And he urged national unity and girded his countrymen for a new battle in the Middle East that could outlast his presidency.

"As Americans, we welcome our responsibility to lead. From Europe to Asia - from the far reaches of Africa to war-torn capitals of the Middle East - we stand for freedom, for justice, for dignity," he said.

"Tonight, I ask for your support in carrying that leadership forward."

The president also insisted that America was not alone in its fight -- arguing that "America will lead a broad coalition to roll back this terrorist threat." (AFP)