Pentagon chief Jim Mattis said Friday he cannot confirm whether or not Islamic State (IS) militant group's chief Abu Bakr al-Baghdadi is dead, after reports from Syria that he had been killed.

"If we knew, we would tell you -- right now, I can't confirm or deny it," Mattis said.

"Our approach is we assume he's alive until it's proven otherwise, and right now I can't prove it otherwise."

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

There was no official confirmation or denial of the news on IS-run social media outlets.