WASHINGTON (AFP) - President Barack Obama will vow in a national address on Wednesday to target the Islamic State (IS) with air strikes "wherever they exist" in a sign he plans to attack the jihadists inside Syria for the first time.
According to excerpts of his speech released by the White House, Obama will also pledge to lead a broad coalition to fight IS and work with "partner forces" on the ground in Syria and Iraq.
But he insisted that US combat troops will not be sent to fight on foreign soil as part of the operation.
The speech will mark a major US escalation, despite Obama having devoted much of his presidency to pulling America out of wars in the Middle East and avoiding new foreign entanglements.
"This counter-terrorism campaign will be waged through a steady, relentless effort to take out ISIL wherever they exist using our air power and our support for partner forces on the ground," Obama says in the excerpts, referring to the Islamic State in Iraq and Levant, which the IS is also known.
"Tonight, with a new Iraqi government in place, and following consultations with allies abroad and Congress at home, I can announce that America will lead a broad coalition to roll back this terrorist threat."
"Our objective is clear: we will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy," he said.
The President will style the new US operation as an extension of long-running counter-terrorism strategies in Yemen and Somalia, where air power and occasional special forces missions have targeted Islamic radical groups.
He also made a clear distinction between the huge land wars launched by his predecessor George W. Bush, in the aftermath of the September 11 attacks, 13 years ago on Thursday.
"I want the American people to understand how this effort will be different from the wars in Iraq and Afghanistan. It will not involve American combat troops fighting on foreign soil," he says.
Earlier, Obama spoke to Saudi King Abdullah, underlining Riyadh's crucial role in an anti-IS front, and reached out to lawmakers to seek Congressional support for his plans.
France offered Obama a well-timed boost, saying it was prepared to join an already launched US air campaign against IS targets in Iraq if necessary.
In Iraq, meanwhile, Secretary of State John Kerry promised that the country's armed forces, some of which fled when IS marched into western Sunni districts, would be "reconstituted and trained and worked on."
Boost for Syrian rebels
Obama and Abdullah "agreed on the need for increased training and equipping of the moderate Syrian opposition," the White House said in a statement.
"Both leaders agreed that a stronger Syrian opposition is essential to confronting extremists like ISIL as well as the Assad regime, which has lost all legitimacy."
The call represented a potentially significant development in the crisis, as Obama for months opposed sending lethal weapons into the Syrian civil war, citing the difficulty of identifying true moderates and fearing they could end up in the hands of extremists.
In a related development, Obama also freed up US$25 million (S$32 million) to equip Iraqi government and Kurdish troops to fight IS.
Though Obama has told Americans he will not send ground troops back into combat, he has not ruled out sending US trainers to help prepare Iraqi forces - stood up at the cost of billions of US taxpayer dollars after the 2003 US invasion.
Kerry praised the new Iraqi government of Prime Minister Haidar al-Abadi, which Washington says has the potential to offer the "inclusive" rule across sectarian divides that the former administration of Nuri al-Maliki failed to provide.
But the sectarian stew facing Abadi was underlined when bombs killed 19 people in Baghdad during Kerry's visit.
The Pentagon said the US military had now conducted 154 strikes against IS in Iraq, and destroyed 212 targets, including armed vehicles and weapons systems.
The plan Obama will lay out on Wednesday will be open-ended and the campaign could outlast his presidency, which ends in January 2017.
"I think the American people need to expect that this is something that will require a sustained commitment," said White House spokesman Josh Earnest.
The address will come at a poignant time - on the eve of the 13th anniversary of the September 11 attacks, when Islamic radicalism on a mass scale scorched the US homeland for the first time.
Obama, who has seen his personal approval ratings and public confidence in his foreign policy tumble, may seek to exploit a seam of increasingly hawkish public opinion on Syria.
A new Wall Street Journal/NBC News poll suggests the US public was increasingly hawkish about IS, with two-thirds of those asked saying they backed military operations against the group.
The White House insists that IS, despite having beheaded two US journalists, does not yet pose an imminent threat to the US homeland.
But officials worry the group could eventually send some of its legions of foreign fighters, armed with Western passports, to stage attacks in America.
Obama previewed his speech in a meeting with top congressional leaders on Tuesday, and then Wednesday huddled with defense and intelligence chiefs in the White House Situation Room.
Officials said he believes he has sufficient authority to carry out his new strategy without asking Congress for a new war authorisation.
But he wants lawmakers to vote on US$500 million in funding for Syrian rebels - which he first requested in June - before they leave town soon ahead of November's mid-term elections.
Republicans have used Obama's public deliberation over what to do about IS to drive a narrative that the president is weak and disengaged.
"Our president must understand we are at war and that we must do what it takes, for as long as it takes, to win," hawkish former vice president Dick Cheney said Wednesday.