BEIRUT — Russia said today it was verifying whether it had killed the leader of the Islamic State group in an airstrike targeting a meeting of IS leaders just outside the group’s de facto capital in Syria, dealing a potentially severe blow to the extremist group as it fights to hang on to its strongholds in Syria and Iraq.

The Russian Defense Ministry claimed that Abu Bakr al-Baghdadi was killed in a Russian strike in late May along with other senior group commanders, adding that the information about his death was still “being verified through various channels.”

Asked later about that claim at a news conference in Moscow, however, Russian Foreign Minister Sergey Lavrov said: “I don’t have a 100-percent confirmation of the information.”

There had been numerous reports in the past of al-Baghdadi being killed but they did not turn out to be true. The IS leader last released an audio on Nov. 3, urging his followers to keep up the fight for Mosul as they defend the Iraqi city against a major offensive that began weeks earlier.

The spokesman for the U.S.-led anti-IS coalition said in a statement today 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 Syria’s Raqqa. Both are under attack by various groups who are fighting under the cover of airstrikes by the U.S.-led coalition.

As the militants take a pounding in their eroding strongholds, U.S. 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 responsibility for 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 responsibility for the June 3 London attack that killed eight people. Both attacks would have taken place after al-Baghdadi’s alleged killing.

The claim of al-Baghdadi’s possible demise also 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.