Moscow said on Friday its forces may have killed Islamic State leader Abu Bakr al-Baghdadi in an air strike near the Syrian city of Raqqa last month.

There was no initial independent confirmation that Baghdadi had been killed. The U.S.-led military coalition fighting against Islamic State said it could not confirm the death, and several Iraqi officials told Reuters they were skeptical.

The secretive Islamic State leader has frequently been reported killed or wounded since he declared a caliphate to rule over all Muslims from a mosque in Mosul in 2014, after leading his fighters on a sweep through northern Iraq.

If the report does prove true, it would be one of the biggest blows yet to Islamic State, which is trying to defend its shrinking territory against an array of forces backed by regional and global powers in both Syria and Iraq.

The Russian Defense Ministry said on its Facebook page that it was checking information that Baghdadi was killed in the strike, which it launched after receiving intelligence that a meeting of Islamic State leaders was being planned.

“On May 28, after drones were used to confirm the information on the place and time of the meeting of IS leaders, between 00:35 and 00:45, Russian air forces launched a strike on the command point where the leaders were located,” the statement said.

“According to the information which is now being checked via various channels, also present at the meeting was Islamic State leader Abu Bakr al-Baghdadi, who was eliminated as a result of the strike,” the ministry said.

The last public video footage of Baghdadi shows him dressed in black clerical robes declaring his caliphate from the pulpit of Mosul’s mediaeval Grand al-Nuri mosque back in 2014.

Born Ibrahim al-Samarrai, Baghdadi is an Iraqi in his mid-forties, who broke away from al Qaeda in 2013 after years participating in the insurgency against U.S. forces in Iraq and the Iraqi government. The U.S. State Department has offered a $25 million reward for information leading to his arrest.

A number of senior IS figures have been killed in air strikes or special forces raids since the United States launched its campaign against the group in 2014, including Baghdadi’s deputy Abu Ali al-Anbari, the group’s “minister of war” Abu Omar al-Shishani, and its media director Abu Muhammad al-Furqan.