As the battle for Mosul intensifies, ISIS has released an audio tape purportedly from its leader Abu Bakr al-Baghdadi, promising jihadi victory.

"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, according to the recording released online by ISIS' media wing on Thursday.

He urged ISIS fighters to "turn the nights of the unbelievers into days, to wreak havoc in their land and make their blood flow as rivers".

"Know that the value of staying on your land with honour is a thousand times better than the price of retreating with shame," he said.

"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."

If it is genuine, the recording is the first message released by Baghdadi since December 2015.

It comes as Christian Aid urged all sides to respect international law and allow civilians trapped in Mosul to flee.

More than 18,000 have left areas around the city since a major offensive to retake Mosul – ISIS' last stronghold in Iraq – led by Iraqi troops and coalition air forces began on October 17.

Up to 1.5 million people are believed still to be trapped, however.

A spokesperson for a Christian Aid partner organisation in Iraq said of civilians left in the city: "They are terrified of the shelling, they are trapped at home, they cannot leave the city because of ISIL."

Frances Guy, Christian Aid's head of region for the Middle East said, "Civilians are caught in the middle of an increasingly complicated conflict with different forces on all sides of the city and little concern for humanitarian principles".

She welcomed the liberation of and "return of the sound of church bells" to a number of Christian villages and towns in the region, but added: "The charred remains of the church at Qaraqosh are a symbolic reminder of the damage that has been done to the multi-religious fabric of Iraq over the last few years.

"The liberation of Christian, Shabak and Turkmen towns highlights the multi-religious make up of Iraq. The opportunity to rebuild a lost harmony must not be thrown away in a potential rush for revenge."