He also called on Islamic State 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.

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."

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 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 Islamic State two years ago in Iraq and neighbouring Syria.

Baghdadi told Islamic State'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.''

Islamic State 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 Shi'ite 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 Shi'ite 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.''

Islamic State's leader 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 airstrikes.