BEIRUT (AP) — Russia said Friday 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.