By Steve Holland and Roberta Rampton
WASHINGTON (Reuters) - President Barack Obama will vow on Wednesday to attack Islamic State militants "wherever they exist" in a speech to Americans expected to lay out a strategy for expanding U.S. air strikes in Iraq and, for the first time, striking targets in Syria.
Obama will stress in his 9 p.m. (0100 GMT Thursday) speech from the White House that he will not send U.S. combat troops to fight the al Qaeda-like group, and that the United States will act in concert with a broad coalition including Western allies and Arab states.
Islamic State, which the United States 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 Americans.
"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 will say in his speech, according to excerpts released by the White House.
U.S. officials said 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.
Obama is expected to urge Congress to approve $500 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 U.S. 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 U.S. air strikes in Iraq in the past month, Iraqi and Kurdish forces have halted the Islamic State advance. Obama had signaled his willingness to expand the mission to Syria, where Islamic State has a stronghold.
That is a significant shift for a president who has been reluctant to increase the U.S. 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 Sept. 11, 2001, attacks on New York and the Pentagon.
'DEGRADE AND DESTROY'
"Tonight you will hear ... how the United States will pursue a comprehensive strategy to degrade and ultimately destroy ISIL, including U.S. military action and support for the forces combating ISIL on the ground," a White House official said.
"The president will discuss how we are building a coalition of allies and partners in the region and in the broader international community to support our efforts."
With the speech, Obama is trying to build on the support that has grown among the American public for military action, partly fueled by anger over the beheading of two American journalists in the past month, and also appeal to likely international partners.
Obama spoke to Saudi Arabia's King Abdullah earlier on Wednesday 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 he did not need additional authorization to carry out his plan, but the White House is eager to have their support along with that of the international community.
Although U.S. officials say there is no imminent threat from Islamic State militants against the United States, 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.
MAKING HIS CASE
Obama came close to direct military action a year ago in Syria to support what Washington considers more moderate rebel forces fighting 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 percent of Americans support air strikes in Iraq and 65 percent support using them in Syria, a Washington Post-ABC News opinion poll found. An NBC News-Wall Street Journal poll showed 61 percent said military action against the group was in the interests of the United States.
Obama showed a willingness to intrude militarily into Syrian space with an unsuccessful operation in July to try to rescue American prisoners.
(Additional reporting by Jeff Mason and Susan Heavey; Editing by Cynthia Osterman and Peter Cooney)