This file image made from video posted on a militant website Saturday, July 5, 2014, purports to show the leader of the Islamic State group, Abu Bakr al-Baghdadi, delivering a sermon at a mosque in Iraq during his first public appearance. (Uncredited/AP)

Islamic State leader Abu Bakr al-Baghdadi expressed confidence in victory in his first message after U.S.-backed Iraqi forces started an offensive to take back Mosul, the last major city under control of his group in Iraq.

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 last message purportedly coming from Baghdadi was from December 2015, an audio recording that reassured followers and supporters that airstrikes by Russia and the U.S.-led coalition had failed to weaken the group in Syria.

1 of 48 Full Screen Autoplay Close Skip Ad × The battle for Mosul View Photos Iraqi forces launch their most ambitious fight against the Islamic State. Caption Iraqi forces continue their most ambitious fight against the Islamic State. Iraqi Federal police celebrate in West Mosul. Alaa Al-Marjani/Reuters Buy Photo Wait 1 second to continue.

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 Oct. 17 with air and ground support from a U.S.-led coalition is shaping up as the largest in Iraq since the U.S.-led invasion of 2003.

Mosul has a population of 1.5 million, much more than any of the other cities captured by Islamic State two years ago in Iraq and neighboring Syria.

Baghdadi told the 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.’’

The Islamic State has been retreating since last year in both Iraq and Syria, in the face of a myriad different forces.

In Iraq, it is fighting U.S.-backed Iraqi government and Kurdish forces, and Iranian-backed Iraqi Shiite militias.

In Syria, it is fighting Turkish-backed Syrian rebels opposed to Syrian President Bashar al-Assad, U.S.-backed Kurdish fighters, and Russian- and Iranian-backed Syrian army units loyal to Assad and foreign Shiite militias.

Baghdadi 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.’’

The 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 U.S. airstrikes.

Read more:

Iraqi elite forces enter Mosul more than 2 years after Islamic State seized the city

Trapped in a house with Islamic State fighters

Iraqi Christians, scarred by Islamic State’s cruelty, doubt they will return to Mosul

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