WASHINGTON, July 21 (Xinhua) -- U.S. Defense Secretary Jim Mattis said on Friday that he believed Islamic State (IS) leader Abu Bakr al-Baghdadi was still alive.

"I think Baghdadi's alive," Mattis said here to reporters. "I'll believe otherwise when we know we've killed him."

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

Mattis' remarks came amid multiple local reports which indicated that al-Baghdadi had already been killed.

According to Iraqi news agency al-Sumaria News, the IS had confirmed the death of al-Baghdadi, who was reportedly killed in a Russian airstrike in Syria.

The news reports came a day after Iraqi Prime Minister Haider al-Abadi formally declared that Mosul was liberated from IS after nearly nine months of fierce fighting.

Xinhua could not independently verify the reported death of al-Baghdadi.