The president made the comment as he authorised surveillance flights over Syria, a move seen as a precursor to step up US efforts in the war-torn country to combat the Islamic State, including possible air strikes on its targets, Xinhua reported.
"Rooting out a cancer like IS won't be easy, and it won't be quick," he said in a speech delivered at the annual convention of the American Legion in Charlotte, North Carolina.
He once again ruled out sending back US combat troops to Iraq, where US warplanes are continuing their air raids on the Islamic State targets, which was launched Aug 8.
"Let me say it again, US combat troops will not be returning to fight in Iraq," he said.
"We'll not allow the US to be dragged back into another ground war in Iraq," he added.