Russia claimed Friday it killed the leader of ISIS in an airstrike targeting a meeting of its leaders just outside the group’s de facto capital in Syria.

The Russian Defence Ministry said Abu Bakr al-Baghdadi was killed in a Russian strike in late May along with other senior group commanders.

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

The spokesman for the U.S.-led anti-ISIS coalition said in a statement Friday he could not confirm the Russian claim.

The report of al-Baghdadi’s death comes as ISIS 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.

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 ISIS-controlled, Euphrates River valley near the Iraqi border.

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.

If confirmed, it would mark a major military success for Russia, which has conducted a military campaign in support of Syrian President Bashar Assad since September 2015.

The Defence Ministry said the air raid on May 28 that targeted an ISIS meeting held on the southern outskirts of Raqqa in Syria also killed about 30 mid-level militant leaders and about 300 other fighters.

The ministry said the ISIS leaders were gathered to discuss the group’s withdrawal from Raqqa, the group’s de facto capital. It said the military began planning the hit after getting word that the group’s leadership was to meet in order to plan ISIS’s exit to the south.

The Russian military sent drones to monitor the area and then dispatched a group of Su-34 bombers and Su-35 fighter jets to hit the ISIS gathering.

“According to the information that is being verified through various channels, ISIS leader Abu Bakr al-Baghdadi also attended the meeting and was killed in the airstrike,” the military said in a statement.

The Defence Ministry added that it had warned the U.S. of the coming strike.

Syrian opposition activists reported airstrikes on May 28 south of Raqqa that killed more than a dozen people.

The Britain-based Syrian Observatory for Human Rights, which tracks Syria’s war, said airstrikes on the road linked the villages of Ratla and Kasrat killed 18 people while the activist-operated Raqqa is Being Slaughtered Silently said 17 civilians were killed in the airstrike on buses carrying civilians.

The Observatory said the dead included 10 ISIS members. It did not elaborate at the time.

The Russian ministry said that among other militant leaders killed in the raid were ISIS leaders Abu al-Khadji al-Mysri, Ibrahim al-Naef al-Khadj and Suleiman al-Shauah.

Al-Baghdadi declared a caliphate in Syria and Iraq in June 2014 days after his fighters captured Mosul, the largest city they ever held. The group has since horrified the world with its atrocities in areas they held as well as attacks they claimed around the world that killed hundreds.

Al-Baghdadi is a nom de guerre for a man identified as Ibrahim Awwad Ibrahim Ali al-Badri al-Samarrai. The U.S. is offering a $25 million reward for information leading to his death or capture.

Alexei Pushkov, the head of the committee for information policies at the upper house of the Russian parliament tweeted that “if confirmed, al-Baghdadi’s death will be a powerful blow to ISIS. It has been retreating on all fronts, and the death of its leader will accelerate its demise.”