Pentagon chief Jim Mattis said Friday that he believes Islamic State chief Abu Bakr al-Baghdadi is still alive, following various claims he was dead.

"I think Baghdadi's alive... and I'll believe otherwise when we know we've killed him," Mattis told Pentagon reporters. "We are going after him but we assume he is alive."

The Britain-based Syrian Observatory for Human Rights, a longtime conflict monitor, last week said it had heard from senior IS leaders in Syria's Deir Ezzor province that Baghdadi was dead.