But U.S. officials have said as recently as August that they believed al-Baghdadi was alive.
The voice purporting to be al-Baghdadi's also urged followers to intensify attacks on the "headquarters" and "media centers" of perceived enemies.
The apparent IS leader called on "soldiers of the caliphate" to pursue their "jihad", or holy war, and attacks. "Do not lay down your arms", Baghdadi said in the recording.
According to a report in The Telegraph, Baghdadi said "the United States was no longer the world's only superpower and gave North Korea's threats as evidence of American decline, as well as Russia's domination of Syrian peace talks in Kazakhstan".
"With God's will and his strength, we are staying determined, patient.The abundance of killing will not stop us", Baghdadi said in the audio recording.
Last week Iraqi forces backed by paramilitary units and coalition warplanes launched a push up the valley. The latter, released first on Telegram -and then on Youtube, download sites, social networks, etc - contains a message from the head of Daech, Abu Bakr al-Baghdadi.
The date of the 46-minute recording, released via the IS-linked Al-Furqan news organization on September 28, was not clear.More news: Facebook sends team to Puerto Rico to get island back online
More news: Gold stays below $1300/oz as dollar rises after hawkish Yellen comments
More news: GOP Leaders Pull Latest Health Care Repeal Effort
The alliance of Kurdish and Arab fighters worked for months to encircle Raqa, which had become a byword for the worst of Daesh's atrocities during its years under the militant rule.
"He purposely used references to current events, like North Korea and Syria, to refute the allegation that Russian Federation had killed him in that air strike in Raqqa back in May", Hoffman told the Fox News "Fox and Friends" program.
Baghdadi also echoes the message of another ISIS leader, Abu Mohammad al-Adnani, who declared that holding territory mattered less than the will to fight.
A USA counterterrorism chief has warned that ISIS' ability to "reach globally is still largely intact". "While we have no reason to doubt its authenticity, we do not have verification at this point".