Defense Secretary Jim Mattis said Friday that he believes Islamic State leader Abu Bakr al-Baghdadi is still alive following weeks of speculation over his death.

Mattis said he has seen no evidence that Baghdadi is dead, although Russia claimed in June it killed him during an airstrike in Syria a month earlier.

The Pentagon has said the group's "caliph," who last appeared publicly in 2014 in Mosul, does not appear to be involved in command-and-control operations over Islamic State forces fighting in Iraq and Syria.

"I think Baghdadi's alive," Mattis said. "I think that he is alive, and I'll believe otherwise when we know we've killed him."

Mattis said it is not clear exactly what role Baghdadi is playing in Islamic State, whether it is operational, strategic, or spiritual. The group has been under increasing pressure, with its de facto capital of Raqqa, Syria under assault and Mosul liberated by Iraqi forces and the U.S.-led coalition.

"For obvious reasons, we're trying to track him pretty closely because he's declared war on us, so he is going to reap his reward for it," Mattis said.