US Defense Secretary Jim Mattis assumes that Islamic State leader Abu Bakr al-Baghdadi is still alive.
Earlier this month, the Syrian Observatory for Human Rights said it had "confirmed information" that Baghdadi had been killed, but Western and Iraqi officials have been skeptical.
"I think that he's alive, and I'll believe otherwise when we know we've killed him. But we're going after him ... we assume he's alive," Mattis told reporters at the Pentagon on Friday.
Earlier on Friday Nicholas Rasmussen, the director of the US Counter-Terrorism Center, said: "I've seen nothing that would lead me to believe that the leader of ISIS (Islamic State) has been removed from the battlefield."
"We know a good bit. We just don't have information that would confirm his death."