The United States Secretary of Defense James Mattis said Friday that he believes Islamic State chief Abu Bakr al-Baghdadi is still alive, following various claims he was dead.

"I think Baghdadi's alive... and I'll believe otherwise when we know we've killed him," Mattis told Pentagon reporters.

"We are going after him, but we assume he is alive," he added.

There have been persistent rumors that Baghdadi has died in recent months.

The Russian Defense Ministry said in mid-June it is checking on information indicating that Daesh leader Abu Bakr al-Baghdadi was reportedly killed in a Russia-led airstrike in Syria.

According to the Russian Defense Ministry, the Russian Su-34 aircraft and an Su-35 multirole fighter carried out airstrikes near the Daesh (IS, formerly ISIS/ISIL) stronghold of Raqqa in northern Syria on May 28.

The strikes targeted a meeting of high-ranking Daesh chiefs where al-Baghdadi was reportedly present, the Russian Defense Ministry said.

The Britain-based Syrian Observatory for Human Rights, a longtime conflict monitor, last week said it had heard from senior Daesh leaders in Syria's Deir Ezzor province that Baghdadi was dead, AFP reported.

With a $25 million US bounty on his head, Baghdadi has kept a low profile but was rumored to move regularly throughout IS-held territory in Iraq and Syria.

The Iraqi native has not been seen since making his only known public appearance as "caliph" in 2014 at the Grand Mosque of Al-Nuri in Mosul, which was destroyed in the battle for Iraq's second city.