Mattis told reporters at the Pentagon on Friday he believed the leader of the terrorist group also known as ISIS, ISIL and Daesh is alive and serving a role in the group despite Russian reports that he had been killed.
"I think Baghdadi is alive," Mattis said. "I'll believe otherwise when we know we have killed him. We are going after him."
In June Russia investigated a May 28 airstrike on a suburban Raqqa, Syria, command post that killed up to 30 Islamic State leaders and 300 troops. The Russian ministry said al-Baghdadi was present "and was eliminated in the strike," adding the information was "being verified via different channels."
Reports of al-Baghdadi's death regularly surface, but the May airstrike was the first time Russia acknowledged it may have killed the terrorist group's top leader.
"Until I see his body, I am going to assume he is alive," Mattis said Friday.
The Syrian Observatory for Human Rights also said it had obtained information from senior Islamic State commanders stating that al-Baghdadi was killed in the Deir al-Zour Province and Pentagon officials said he did not seem to be directing the group's daily actions.
Mattis said he believed al-Baghdadi still "had a role to play" in the Islamic State organization, stating he may have assumed a role as a spiritual or propaganda leader.
Other U.S. officials have challenged reports of al-Baghdadi's death, but Lt. Gen. Stephen Townsend, who is the head of the U.S. and coalition effort against ISIS in Iraq and Syria, said there was not sufficient evidence to prove whether he is alive or dead.
"Despite all the helpful reports to us from every source imaginable, I'm unable to confirm or deny where he is or whether he is alive or dead," Townsend said last week. "Let me just say for the record, my fervent hope is it is the latter.'