The leader of the so-called Islamic State group, 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.
Mosul is the last major city under control of his group in Iraq.
He also called on IS 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," Baghdadi said in an audio recording released online by supporters.
The authenticity of the 31-minute-long recording could not be verified.
The previous message purportedly coming from Baghdadi was from December 2015, an audio recording that reassured followers and supporters that airstrikes by Russia and the US-led coalition had failed to weaken the group in Syria.
Baghdadi, an Iraqi whose real name is Ibrahim al-Samarrai, called on the population of Mosul's Nineveh province "not to weaken in the jihad" against the "enemies of God".
The battle that started on 17 October with air and ground support from a US-led coalition is shaping up as the largest in Iraq since the US-led invasion of 2003.
Mosul still has a population of 1.5 million people, much more than any of the other cities captured by IS two years ago in Iraq and neighbouring Syria.
Baghdadi told IS 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."
IS 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 rebels opposed to Syrian President Bashar al-Assad, US-backed Kurdish fighters as well as Russian and Iranian-backed Syrian army units loyal to Assad and foreign Shia militias.
The United Nations fears that IS is forcibly gathering people in and around Mosul for possible use as human shields against advancing Iraqi forces.
The UN has cited reports of IS kidnapping thousands of people for use as human shields, and also of the jihadists executing nearly 300 people in the Mosul area since 25 October.
