BEIRUT — Uncertainty and confusion surrounded the fate of the head of the Islamic State on Friday as Russia announced it may have killed him in an airstrike targeting a meeting of leaders just outside the group’s self-declared capital in Syria.

But U.S. officials said there was no definitive proof of his death.

The demise of Abu Bakr al-Baghdadi would be a severe blow to the extremist group as it fights to hang on to its strongholds in Syria and Iraq, although it was not clear how much operational control he retains over the organization whose capabilities keep evolving on the battlefield and beyond.

Apart from Moscow’s claim that he may have been killed in the May 28 airstrike along with more than 300 militants, there was not much else to back it up. The Russian Defense Ministry said the information about his death was still “being verified through various channels.”

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

A spokesman for the U.S.-led coalition fighting the Islamic State said he would welcome such news but urged caution.

“There have been several past claims of this kind that have been proven false, and we have seen no definitive proof that this report is true either,” Army Col. Ryan Dillon said.

Capt. Jeff Davis, a Pentagon spokesman, also said there was no information to corroborate the report.

Al-Baghdadi, believed to be in his mid-40s, last released an audio message Nov. 3, urging his followers to keep up the fight for Mosul as they defended the Iraqi city against a major offensive that had begun weeks earlier.

The latest report of his death comes amid major setbacks for the Islamic State, having lost significant territory on both sides of its so-called caliphate in Syria and Iraq.

The group is fighting for survival in a few remaining neighborhoods in western Mosul and is now under attack in Raqqa, its self-styled capital.