Recorded message says "total war" for Iraq’s second city "increases our firm belief all this is a prelude to victory".
ISIL leader Abu Bakr al-Baghdadi has expressed confidence in victory, in his first message after US-backed Iraqi forces started an offensive to take back Mosul, the last major city under control of his group in Iraq.
He also called on Islamic State of Iraq and the Levant (ISIL) fighters to invade Turkey.
"This raging battle and total war, and the great jihad that the state of Islam is fighting today only increases our firm belief, God willing, and our conviction that all this is a prelude to victory," he said in an audio recording released online by supporters on Thursday.
Al Jazeera could not verify the authenticity of the 31-minute-long recording.
Sherifa Zuhur, a visiting scholar at the University of California, Berkeley, told Al Jazeera that with this recording Baghdadi is sending a message to supporters of ISIL, also known as ISIS, to fight to the death.
"It’s a call to martyrdom that says: if you stand in front of the coalition, just stand firm and say ’God is enough for us’," she said.
"On the other hand, it is a statement that ISIL is going to lose its physical territory, for now, and will go back to the condition that it was in, in 2010. It is also a call to those he feels have betrayed the cause of ISIL, a reference to other Sunnis in different groups."
The previous message purportedly from Baghdadi was from December 2015, an audio recording that reassured followers and supporters that air strikes by Russia and the US-led coalition had failed to weaken the group in Syria.
Zuhur said the latest recording was made within the last 10 days.
"We know that because he made reference to a phrase in a speech by al-Maliki about a recent operation in Nineveh province," she said, referring to Nouri al-Maliki, the former Iraqi prime minister.
Baghdadi, an Iraqi whose real name is Ibrahim al-Samarrai, called on the population of Iraq’s Nineveh province "not to weaken in the jihad" against the "enemies of God".
He also called on the group’s suicide fighters to "turn the nights of the unbelievers into days, to wreak havoc in their land and make their blood flow as rivers".
The battle that started on October 17 with air and ground support from a US-led coalition is shaping up as the largest in Iraq since the invasion of 2003.
Mosul still has a population of 1.5 million people, much more than any of the other cities captured by ISIL two years ago in Iraq and neighbouring Syria.
Baghdadi told ISIL’s fighters to "unleash the fire of their anger" on Turkish troops fighting them in Syria, and to take the battle into Turkey.
"Turkey today entered your range of action and the aim of your jihad ... invade it and turn its safety into fear."
ISIL has been retreating since last year in both Iraq and Syria, in the face of a myriad of different forces.
In Iraq, it is fighting US-backed Iraqi government and Kurdish forces, and Iranian-backed Iraqi Shia militias.
In Syria, it is fighting Turkish-backed Syrian fighters opposed to Syrian President Bashar al-Assad, US-backed Kurdish forces as well as Russian- and Iranian-backed Syrian army units loyal to Assad and foreign Shia militias.
Baghadi told his followers to launch "attack after attack" in Saudi Arabia, targeting security forces, government officials, members of the ruling Al Saud family and media outlets, for "siding with the infidel nations in the war on Islam and the Sunna [Sunni Muslims] in Iraq and Syria".
He also said "the caliphate was not affected" by the death of some of its senior commanders, mentioning Abu Muhammad al-Adnani and Abu Muhammad al-Furqan, both killed earlier this year in US air strikes.