Islamic State leader Abu Bakr al-Baghdadi urged his fighters in an audio recording posted online on Thursday not to retreat from the northern Iraqi city of Mosul, but also to invade Turkey and attack Saudi Arabia.

Al-Baghdadi called on his militants to transform into rivers the blood of the Iraqi forces attempting to retake Mosul in Nineveh province.

‘Do not retreat ... this whole war only gives us more faith to fight towards victory which is a promise from God,’ al-Bahgdadi said.

‘To all the strugglers and the people of Nineveh, beware of any weakness ... fight and struggle and face your enemy,’ he added.

 He called on the suicide bombers ‘to confront the enemy and transform their blood into rivers ... transform their days into dark nights.’  ‘This war is your war,’ he said.

Islamic State overran Mosul in mid-2014 at the beginning of a lightning offensive that saw it seize swathes of Sunni Arab northern and western Iraq.

On October 17, government forces, Kurdish troops and Sunni and Shia fighters, backed by US-led airpower, started a long-awaited campaign to liberate Mosul, Islamic State's de facto capital in Iraq.

The Islamic State leader fiercely criticized Turkey.

‘The fighters of the Caliphate, here comes the atheist, Turkish soldiers, ... show them your strength,’ he said in the 31-minute audio recording posted on the Islamic State website al-Furqan.

The recording could not be independently verified by dpa.

Turkey has been backing opposition rebels fighting Islamic State in northern Syria.

‘Turkey remained during our struggle with the supporters of atheism ... seeking to achieve its interests in northern Iraq and on the borders [of Syria],’ al-Baghdadi said.

He said that the airstrikes against Islamic State by the US-led alliance and Russia had failed to weaken the extremist group, and referred to the ongoing attempts to dislodge rebels from the north-western Syrian city of Aleppo.

‘Here is Aleppo facing the harshest crusade ... supported by the atheist Russia amidst the treason of the groups busy with fighting Islamic State for the sake of the interests of their masters from heretic countries,’ al-Baghdadi said.

In the recording, al-Baghdadi accused Saudi Arabia of turning Syria into a secular country, calling on Islamic State militants to attack the kingdom.

‘Not only that, they took part in the alliance to fight Islam {in Syria],’ he said.