WASHINGTON - 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 US airstrikes in Iraq and, for the first time, striking targets in Syria.

Obama will stress in his 9 pm (0100 GMT Thursday) speech from the White House that he will not send US combat troops to fight the al Qaida-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.

"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.

US officials said Obama was opening the door to airstrikes 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.

After more than 150 US airstrikes 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.

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.