Addressing the estimated 5,000 ISIS members holed up in the city, Baghdadi says that "holding your ground in honor is a thousand times better than retreating in disgrace."
The tape, the first recording of Baghdadi released since late 2015
, features the ISIS leader claiming to be "confident of victory" in Mosul, the group's Iraqi stronghold, despite being vastly outnumbered by the approaching troops.
Released by ISIS' media wing al-Furqan, the message has a vicious sectarian tone, with the animosity toward Shia Muslims and other, non-Muslim groups more pronounced than in previous recordings.
The voice, which CNN cannot independently verify as being that of Baghdadi, says that "God's enemies from the Jews, Christians, atheists, Shiites, apostates and all of the world's infidels have dedicated their media, money, army and munitions to fight Muslims and jihadists in the State of Nineveh after they witnessed it become one of the bases of Islam and one of its minarets under the Caliphate."
Nineveh is an ancient name for the modern-day Mosul region.
The audio, nearly 32 minutes long, references Turkey's massing of armaments along the Iraq-Turkey border, suggesting that the recording was made very recently.
Along with rallying ISIS-affiliated fighters across the globe, he warns jihadists against "conflict and disagreement."
A US defense official said the Pentagon hasn't confirmed the authenticity of the recording.
"But it is quite clearly an effort on the part of (ISIS) to communicate to their fighters," spokesman Col. John Dorrian said. "This is probably excellent evidence that their command and control and ability to communicate directly with their fighters and control them has been severely reduced."
The leader of the Islamist group first came into the public eye
with a sermon delivered at the Great Mosque of al-Nuri in Mosul in July 2014.
His whereabouts are unknown, with his location a matter of utmost secrecy for the terror group.
US officials have said that one scenario is that he has moved from one place to another within ISIS' shrinking so-called caliphate
to avoid detection -- and that they would attack his location if they knew where he was.
ISIS has lost many figures from among its senior ranks in the past two years, mostly to US airstrikes, including official spokesman Mohammad al-Adnani in August
The most recent audio message attributed to Baghdadi surfaced online in December 2015
, mocking the Western coalition aligned against him, threatening Israel and trying to boost his own forces' morale.
Where did ISIS -- and Baghdadi -- come from?
The group's roots are in the Sunni terror group al Qaeda in Iraq (AQI), started in 2004 by Jordanian Islamist Abu Musab al-Zarqawi. It was a major player in the insurgency against the US-led forces that toppled Saddam Hussein in 2003, and against the Shiite-dominated government that eventually replaced Hussein.
After Zarqawi was killed in a US airstrike in 2006, Egyptian Abu Ayyub al-Masri took over and announced the creation of the Islamic State in Iraq. The words "and Syria" would come later.
By 2006, ISIS controlled much of western Iraq's Anbar province. But then in 2008, a surge of US troops, with the help of Sunni tribesmen who were at odds with al Qaeda, largely defeated the group in Iraq.
Masri was killed in a 2010 US-Iraqi operation. That opened the door to Baghdadi, who is also known to the United States as Ibrahim Awad Ibrahim Al Badry.
Before taking control of ISIS, Baghdadi was taken into US custody in Falluja in the Anbar province on February 4, 2004, according to a Pentagon statement.
There have been mixed reports about when and why he was detained. He was held until the following December at a prison at Camp Bucca, according to the Pentagon. A review board recommended his "unconditional release," the statement said.
While the Pentagon said Baghdadi was from Falluja, other reports have said he is from Samarra, north of Baghdad.