Russia is trying to verify reports that its military has killed the leader of the Islamic State group in an air strike targeting a meeting of militant leaders.
The Defence Ministry claimed that Abu Bakr al-Baghdadi was killed in late May with other senior group commanders just outside the group's de facto capital in Syria, adding that information is "being verified through various channels".
Foreign minister Sergey Lavrov said: "I don't have a 100% confirmation of the information."
There have been previous reports of al-Baghdadi being killed but they did not turn out to be true.
The IS leader last released an audio recording on November 3, urging his followers to keep up the fight for Mosul as they defend the Iraqi city against a major offensive.
A spokesman for the US-led anti-IS coalition said he could not confirm the Russian claim.
The report of al-Baghdadi's death comes as IS suffers major setbacks in which they have lost wide areas of territory and both of their strongholds - Mosul in Iraq and Raqqa in Syria.
Both are still under attack by various groups fighting under the cover of air strikes by the US-led coalition.
As the militants take a pounding in their eroding strongholds, US officials and Syrian activists say many commanders have fled Mosul and Raqqa in recent months for Mayadeen, a remote town in the heart of Syria's IS-controlled Euphrates River valley near the Iraqi border.
Their relocation could extend the group's ability to wreak havoc in the region and beyond for months to come.
Most recently, the group claimed attacks in Iran's parliament and a shrine to revolutionary leader Ayatollah Ruhollah Khomeini in Tehran, killing at least 17 people and injuring more than 50.
It also claimed the June 3 London attack that killed eight people. Both attacks would have taken place after al-Baghdadi's alleged killing.
Mr Lavrov said that if al-Baghdadi's death is confirmed, its importance must not be overestimated.
He said that "past examples of similar actions to strike the leadership of terrorist groups were presented with much enthusiasm and pomp, but the experience shows that those structures later regained their capability".
The claim of al-Baghdadi's possible death comes nearly three years to the day after he declared himself the leader of an Islamic caliphate in Iraq and Syria, from a historic mosque in Mosul.
If confirmed, his death would mark a major military success for Russia, which has conducted a military campaign in support of Syrian President Bashar Assad since September 2015.
It is not clear who would replace al-Baghdadi if he was killed - the group has lost many of its senior commanders in US-led air strikes, including Fadhil Ahmad al-Hayali, said by US officials to be the second in command of the group.
He was killed in an August 2015 air strike by the US in Iraq.
Another top figure was Abu Ali al-Anbari, the extremist group's leading finance official who was killed last year.
One of the best known IS commanders, Abu Mohammed al-Adnani, the spokesman who frequently released audio messages calling for stepped-up attacks, was also killed last year.
Moscow's Defence Ministry said the air raid on May 28 that targeted an IS meeting on the southern outskirts of Raqqa killed about 30 mid-level militant leaders and 300 other fighters.
The ministry said the IS leaders were gathered to discuss the group's withdrawal from Raqqa, the group's de facto capital.