Yet he ruled out large-scale US ground combat operations reminiscent of Iraq and Afghanistan.

"I'm convinced that the United States should not get dragged back into another prolonged ground war," the president said on Wednesday at the White House as he set Congress on a path to its first war-powers vote in 13 years.

Despite his words of reassurance, initial reaction in Congress amounted to bipartisan skepticism, with much of the dissatisfaction centered on his attempt to find a political middle ground with respect to ground forces.

Republicans expressed unhappiness that he had chosen to exclude any long-term commitment of ground forces, while some Democrats voiced dismay that he had opened the door to deployment at all.

Sen John McCain, a leading Republican, also said Obama had ruled out air support for U.S.-trained rebels battling Syrian President Bashar Assad, adding, "That's immoral."

Under Obama's proposal, the use of military force against Islamic State fighters would be authorized for three years, unbounded by national borders. The fight could be extended to any "closely related successor entity" to the Islamic State organization that has overrun parts of Iraq and Syria, imposed a stern form of Sharia law and killed several hostages it has taken, Americans among them.

"Make no mistake. This is a difficult mission," Obama said in seeking action against a group that he said threatens America's own security. He said it will take time to dislodge the terrorists, especially from urban areas. "But our coalition is on the offensive. ISIL is on the defensive, and ISIL is going to lose."

The 2002 congressional authorization that preceded the American-led invasion of Iraq would be repealed under the White House proposal, a step some Republicans were unhappy to see. But a separate authorization that was approved by Congress after the Sept. 11, 2001, terror attacks would remain in force, to the consternation of some Democrats.

At the heart of the debate, the struggle to define any role for American ground forces is likely to determine the outcome of the administration's request for legislation.     

White House spokesman Josh Earnest said the proposal was intentionally ambiguous on that point to give the president flexibility, although the approach also was an attempt to bridge a deep divide in Congress.

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