The IS leader Abu Bakr al-Baghdadi could still be alive, a top western coalition general has said.
Major General Felix Gedney said "we don’t know exactly where he is" and it's still unclear whether has been killed.
"We hear reports of him being alive but we can’t confirm that," he said.
"We don’t know exactly where he is, when we find him you’ll know about him because we’ll deal with him."
Russian forces claimed in June last year that there was a "high probability" that al-Baghdadi had been killed alongside other senior militants just outside Raqqa, Syria.
However, leading western commanders have thrown doubt in those claims.
Major Gedney, who is effectively number two in the US-led Coalition military campaign against Islamic State in Iraq and Syria, said the battle against IS has seen the fiercest fighting since World War Two.
He said local forces had "remarkable success" in "bitter" battle against a "vicious" enemy.
"I don’t think you’ve seen fighting the like of which we’ve seen in Iraq and Mosul since the Second World War," he said.
"What we’re fighting against is a really vicious enemy, an enemy that would hold civilians on the battlefield, shoot them if they tried to leave, use them as human shields to protect them on the battlefield, and that’s really difficult to deal with."
Local forces were now moving to clear out remaining pockets of IS fighters, he said.
However, he warned the group will still pose a threat in Iraq and Syria until they impose a "lasting defeat" that allow residents to return home and rebuild stability.