The head of the Islamic State released a message encouraging forces in the fight for Mosul, which they are defending against Iraqi forces, according to a U.S. organization that monitors militant activity online said Thursday.

Abu Bakr al-Baghdadi spoke in a more than half an hour long audio recording released on Wednesday. According to Reuters, the recording showed his confidence in a victory.

Supporters released the message online. Reuters reported that the authenticity could not be verified.

"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," al-Baghdadi reportedly said in the recording.

In the recording, al-Baghdadi rallies his fighters and is calling on them to obey orders while remaining resilient and steadfast. He urges others to carry out attacks in Saudi Arabia and Turkey.

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"Oh you who seek martyrdom! Start your actions! Turn the night of the disbelievers into day," al-Baghdadi says, adding, "totally decimate their territories, and make their blood flow like rivers!"

Reuters reported that this battle is starting to look like the biggest in Iraq since the U.S.-led invasion in 2003.

"Turkey today entered your range of action and the aim of your jihad ... invade it and turn its safety into fear.'' al-Baghdadi said.

He also called on fighters to "respond to all attacks," and to "target all in their media and forces, and all who belong to them."

Reuters added that al-Baghdadi told supporters to launch "attack after attack'' in Saudi Arabia, targeting those who are "siding with the infidel nations in the war on Islam and the (Sunni Muslims) in Iraq and Syria.''

The city of more than one million people and surrounding territory fell to ISIS fighters during a surprise attack in June 2014. Al-Baghdadi visited the city after the takeover, and from inside Mosul, declared an Islamic caliphate that at one point covered nearly a third of Iraq and Syria.

The Associated Press contributed to this report.

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