US President Barack Obama has ordered a "relentless" war against the Islamic State, including air strikes in Syria and expanded operations in Iraq to "destroy" the marauding jihadist army.
Obama - who hoped to be remembered for ending wars, not starting an open-ended escalation in the Middle East - also pledged to bolster Iraqi forces and increase military assistance to Syria's opposition.
In a prime-time televised address on Tuesday, US time, Obama said the Islamic State (also known as ISIL), which 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.
"They execute captured prisoners. They kill children. They enslave, rape and force women into marriage," he said.
Such barbarity could bring only one response, Obama said.
"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 will not hesitate to take action against ISIL in Syria, as well as Iraq," he said.
"This is a core principle of my presidency: if you threaten America, you will find no safe haven."
A senior US official left no doubt that Obama's words represented an order for military operations in Syria, despite his long-held reluctance to get embroiled in the country's civil war.
"There will be action in Syria," the official said, but declined to say when the first strikes would take place. "We are not going to telegraph our punches."
But, seeking to preserve his doctrine of replacing wars using bogged-down land armies with limited but lethal arms-length warfare, Obama compared the new conflict to anti-terrorism operations in Somalia and Yemen.
He made clear in the 14-minute address the new front against terrorism would not become a repeat of the ground wars of the past decade.
Instead, Washington will empower local partners such as the Iraqi forces, Kurdish fighters and Syrian rebels to fill in territory opened up by US air power.
Obama also announced the dispatch of another 475 military personnel to help train Iraqi forces to take on the Islamic State, bringing the total number of US troops in the country to 1600.
The president also urged Congress to swiftly approve a $US500 million ($A540.98 million) request for funding for the Syrian opposition training mission he first made in June.
Obama warned that Washington would never work with Syrian President Bashar al-Assad to combat Islamic State, arguing he had lost legitimacy by waging a brutal civil war against his people.
He said he believed he had the power to conduct the campaign against Islamic State under existing anti-terrorism authorisations.
Obama's Republican foes welcomed his robust new anti-Islamic State policy, but many accused him of dithering for weeks over what to do.
"I support the president's plan to train and equip the Iraqi Security Forces and Syrian opposition, but I remain concerned that those measures could take years to fully implement," House Speaker John Boehner said.
But some Democrats in the Senate, some facing tough races in November's congressional elections, distanced themselves from the president's plan.
Senator Mark Udall of Colorado said he worried about the offensive expanding into a land war, which he said would need congressional approval.
"The American people must be assured that we are not pursuing another open-ended conflict in the Middle East, and I will not give this president - or any other president - a blank cheque to begin another land war in Iraq," Udall said.
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