The Syrian Observatory for Human Rights (SOHR) says it has "confirmed information" that Islamic State leader Abu Bakr al-Baghdadi has been killed.
"(We have) confirmed information from leaders, including one of the first rank, in [Isis territory] in the eastern countryside of Deir Ezzor," director of the UK-based group Rami Abdulrahman told Reuters on Wednesday (NZ time).
UK-based SOHR is a respected human rights and war crimes monitor, utilising sources on the ground in Syria with a credible track record reporting on the civil war.
But SOHR's sources did not say when Baghdadi supposedly died, or how.
US, Kurdish and Iraqi sources were all quick to say they could not corroborate the SOHR report.
There has been no confirmation from Isis' official news agency Amaq.
Russia's defence ministry said it may have killed Baghdadi in June, when one of its air strikes hit a gathering of Islamic State 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.
"We take any report of this nature with a large dose of salt," Sebastian Gorka, deputy assistant to US President Donald Trump, said in an interview with Fox News after the SOHR claim surfaced.
"We will verify it. We will look at the intelligence available, and we will give a statement when we have the requisite facts."
Reuters could not independently verify Baghdadi's death.
Baghdadi's death, which has been frequently reported since he declared a caliphate from a mosque in the Iraqi city of Mosul in 2014, is one of the biggest blows yet to the jihadist group, which is trying to defend shrinking territory in Syria and Iraq.
Last week the Iraqi army officially declared Isis' de facto Iraqi capital of Mosul liberated from the militants, marking the destruction of one half of the so-called caliphate.