Cairo/Beirut: The Syrian Observatory for Human Rights told Reuters on Tuesday that it had "confirmed information" that IS leader Abu Bakr Al Baghdadi has been killed.
The report came just days after the Iraqi army recaptured the last sectors of the northern Iraqi city of Mosul, which Baghdadi's forces overran almost exactly three years ago.
Russia's Defence Ministry said in June that it might have killed Baghdadi when one of its air strikes hit a gathering of IS commanders on the outskirts of the Syrian city of Raqqa.
But Washington said it could not corroborate the death and Western and Iraqi officials have been sceptical.
Reuters could not independently verify Baghdadi's death.
"(We have) confirmed information from leaders, including one of the first rank who is Syrian, in the Islamic State (IS) in the eastern countryside of Deir al-Zor," said Rami Abdulrahman, the director of the British-based war monitoring group.
In Iraq, U.S. Army Colonel Ryan Dillon, spokesman for the U.S.-led coalition fighting S, said he could not confirm the news. Abdulrahman said activists working with him in Deir Al Zor had been told by the IS sources that Baghdadi had died, but not when or how.
The sources said Baghdadi had been present in the eastern countryside of Syria's Deir al-Zor province in the past three months.
The Pentagon said it had no information to corroborate the reports.
Kurdish and Iraqi officials also had no immediate confirmation.
Baghdadi's death has been announced many times before, but the Observatory has a record of credible reporting on the Syrian conflict.
IS-affiliated websites and social media feeds have so far said nothing.
The death of Baghdadi, who declared a caliphate from a mosque in Mosul in 2014, would be one of the biggest blows yet to the extremist group, which is trying to defend shrinking territory in Syria and Iraq.
The United States put up a $25 million reward for his capture, the same amount as it had offered for Al Qaeda leader Osama bin Laden and his successor Ayman Al Zawahri.
It is not yet known if anybody will claim the bounty.
The IS leaders killed in Iraq and Syria since the U.S.-led coalition began its air strikes include Abu Ali Al Anbari, Baghdadi's deputy; the group's "minister of war", Abu Omar Al Shishani, a close military adviser to Baghdadi; and Abu Mohammad Al Adnani, one of its most prominent and longest-serving leaders.