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," Mr 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."
Mr 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.
Meanwhile in Iraq, local forces continue to face pockets of resistance from Islamic State in Mosul's Old City four days after the Prime Minister declared victory.
Iraqi army helicopters flew overhead and explosions could be heard on Friday, residents said, while videos of alleged revenge attacks against people detained during the retaking of Mosul underlined future security challenges.
"Three mortars landed on our district," a resident of Faysaliya, in east Mosul, just across the Tigris river, said by telephone.
A few hundred Islamic State fighters swept into Mosul three years ago and imposed a reign of terror after the Iraqi army collapsed.
Iraq's victory in Mosul marks the biggest defeat for Islamic State, which is under siege in the Syrian city of Raqqa, its operational base.
Even though the group's caliphate is crumbling, it is expected to revert to an insurgency and keep carrying out attacks in the West and Middle East.
On Thursday, Human Rights Watch said it had used satellite imagery to verify that a video published on Facebook showing armed men in military uniforms beating a detainee before throwing him from a height and then shooting at him, had been filmed in west Mosul.
The footage shows the men shooting at the body of another man already lying at the bottom of the perch. The footage could not be independently verified.
Reuters / Newshub.