The so-called Islamic State group has released an audiotape which it says is from its leader, Abu Bakr al-Baghdadi.
If true, it would be the first public message from him in more than a year and would dispel rumours that he was dead.
The voice on the audio calls on Iraqis to defend the city of Mosul against the Iraqi army, which is attempting to re-take it from IS.
Baghdadi is believed to be inside Mosul alongside thousands of IS fighters.
It has not been independently verified that the voice in the audio belongs to Baghdadi. There have been repeated rumours of his death through the years, including last year when the military claimed to have hit his convoy.
Mosul, the last IS urban stronghold in Iraq, is where Baghdadi declared a caliphate two years ago.
In the message released early on Thursday, a voice says: “Holding your ground with honour is a thousand times easier than retreating in shame.”
“Do not retreat,” it says. “this total war and the great jihad only increased our firm belief, God willing, and our conviction that this is all a prelude to victory.”
The voice calls on people in Nineveh, the Iraqi province where Mosul is located, to fight the “enemies of God”.
It also calls for more IS sympathisers to attack other countries, saying suicide fighters should “turn the nights of the unbelievers into days, to wreck havoc in their land and make their blood flow as rivers”.
A collection of Iraqi forces backed by an international coalition launched an offensive on Mosul on 17 October, the latest attempt to wrest land back from IS.
Mosul battle: Tape ‘from IS leader Baghdadi’ urges no retreat}