Isis has released an audio message apparently of its leader Abu Bakr al-Baghdadi calling on his fighters not to retreat in Mosul as Iraqi forces ready themselves for a final assault on the city.
Al-Baghdadi called on Isis supporters to fight the "enemies of God" and for the suicide fighters to "turn the nights of the unbelievers into days, to wreck havoc in their land and make their blood flow as rivers."
The message released by the Isis-affiliated Al-Furqan media says: "Holding your ground with honour is a thousand times easier than retreating in shame," he said, according to agencies AFP and Reuters.
"To all the people of Nineveh, especially the fighters, beware of any weakness in facing your enemy."
If confirmed, it would be the first time the Isis leader had been heard from since an air strike on Mosul in March 2015 in which he was rumoured to have been paralysed, with some reports, later denied, that he had been killed.
Meanwhile, Amnesty International has said that militias involved in driving out Isis have been carrying out revenge attacks on men and boys in villages close to Mosul.
The human rights group says people suspected of having ties to the Islamists have been tortured and beaten.
Lynn Maalouf from Amnesty said, according to the BBC: "Only those legally sanctioned to detain and interrogate suspects must be allowed to do so.
"The authorities must rein in the tribal militia fighters responsible and bring them to justice in order to prevent such crimes being repeated in the ongoing Mosul offensive."
In tackling Isis, the Iraqi army has joined forces with Kurdish Peshmerga fighters, Shia militias and Sunni Arab tribesmen, with air and ground support from the multinational coalition.