The death of Baghdadi - if true - would mark another blow to the extremist group days after it was finally driven from the northern Iraqi city of Mosul by Iraqi-led forces. The militants are also under siege in the Syrian city of Raqqa, the self-declared Isis capital, where a US-backed force of mainly Kurdish fighters has been advancing for weeks.
The monitoring group, the Syrian Observatory for Human Rights, is based in Britain and relies on a network of activists in Syria for its reporting. The group's director, who goes by the pseudonym Rami Abdulrahman, told the Washington Post that Isis leaders had confirmed Baghdadi's death to the Observatory's activists in the Syrian province of Deir al-Zour.
"We don't know when. We don't know how," Abdulrahman said.
A post on the Syrian Observatory's website said it "confirmed information . . . about the death" of Baghdadi, whose real name is Awwad Ibrahim Ali al-Badri al-Samarrai.
Baghdadi's apparent death or capture have been asserted several times over the past decade as his breakaway al-Qaeda faction gradually formed into the core of Isis. But the various reports later proved false or could not be substantiated.
Last month, Russia said it was looking into claims that one of its airstrikes in late May, outside Raqqa, killed Baghdadi. The Russian military later said it could find no clear evidence of Baghdadi's death.
In a video conference with reporters today, Lieutenant General Stephen Townsend, the top US commander in Iraq, said he had received reports since May "that suggested he was not killed there by the Russians, but I don't know".
"We've heard all kinds of reporting that he's alive, that he's dead," Townsend said. "I hope he's deader than a doornail. If he's not, as soon as we find out where he is, he will be."
As far back as December 2012, Iraqi officials claimed that they had captured Baghdadi in Baghdad. Iraqi commanders later said the person held was not Baghdadi.
