US cannot prove IS chief is dead: Mattis

The United States cannot prove that Islamic State leader Abu Bakr al-Baghdadi is dead, US Defence Secretary Jim Mattis says.

US Defence Secretary Jim Mattis says the United States cannot prove that Islamic State leader Abu Bakr al-Baghdadi is dead.

Earlier this week, the Syrian Observatory for Human Rights said it had "confirmed information" that Baghdadi had been killed, but Western and Iraqi officials have been sceptical.

"If we knew, we would tell you. Right now, I can't confirm or deny it," Mattis said. "Our approach is we assume that he's alive until it is proven otherwise, and right now, I can't prove it otherwise."

Mattis added that a ceasefire in southwestern Syria was holding so far.

The United States, Russia and Jordan announced the ceasefire and a "de-escalation agreement" for the southwest after a meeting between US President Donald Trump and Russian President Vladimir Putin at the G20 summit in Hamburg, Germany.

"What is happening further down in the southwest is something that we will support obviously, but right now, we don't have any equity in it, that is a State Department, diplomatic lead, President to President effort," Mattis said.