The Syrian Observatory for Human Rights (SOHR) said on Tuesday that it had "confirmed information" that Daesh leader Abu Bakr al Baghdadi has been killed.
"Top tier commanders from Daesh who are present in Deir Ezzor province have confirmed the death of Abu Bakr al Baghdadi, emir of the Daesh group, to the Observatory," SOHR director Rami Abdel Rahman said.
"We learned of it today but we do not know when he died or how."
— #المرصدالسوري #SOHR (@syriahr) July 11, 2017
Deir Ezzor, in eastern Syria, remains largely under Daesh control even as the group is losing territory elsewhere in the country and in neighbouring Iraq.
Abdel Rahman said Baghdadi "was present in eastern parts of Deir Ezzor province" in recent months, but it was unclear if he was killed in the area or elsewhere.
The US general who heads the coalition fighting Daesh said he had no information that would confirm or deny claims that al Baghdadi was dead.
Lieutenant General Stephen Townsend said he had heard "all kinds of reporting" about Baghdadi's status.
"I don't have a clue," Townsend said in a video call from Baghdad. "Hope he's deader than a doornail. And if he's not, as soon as we find out where he is he will be.
Russia's army said Sukhoi warplanes carried out a 10 minute raid on May 28 at a location near the Daesh stronghold of Raqqa, where group leaders had gathered to plan a pullout from the area.
The US-led coalition fighting the group in Syria and Iraq said at the time it could not confirm whether the Russian strike had killed Baghdadi.
The 46-year-old Iraqi-born leader of Daesh has not been seen in public since making his only known public appearance as "caliph" in 2014 at the Grand Mosque of Al-Nuri in Mosul.
Daesh destroyed the highly symbolic site before Iraqi forces could reach it as they pushed the militant group from Mosul, where Iraq's government formally declared victory on Monday.
With a $25-million US bounty on his head, Baghdadi has kept a low profile and was rumoured to move regularly throughout the Daesh-held territory in the area straddling Iraq and Syria.
His death would be a new blow to the group which is trying to defend shrinking territory in Syria and Iraq.