IRBIL, Iraq (AP) — The leader of the Islamic State group has released a new message encouraging his followers to keep up the fight for the city of Mosul, which they are defending against Iraqi government forces, a U.S. organization that monitors militant activity online said Thursday.

The SITE Intelligence Group said the speech purporting to be from Abu Bakr al-Baghdadi was released in an audio recording, more than half an hour long, late on Wednesday.

In the recording, al-Baghdadi rallies his fighters — especially in Mosul — 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.

"Oh you who seek martyrdom! Start your actions! Turn the night of the disbelievers into day," he says. "Totally decimate their territories, and make their blood flow like rivers!"

The recording was the reclusive al-Baghdadi's first released message to supporters since Iraqi forces launched the decisive battle to retake Mosul, the country's second-largest city, from IS. The message apparently was an attempt to harness the feelings of Sunni disenfranchisement that preceded the 2014 IS takeover of Mosul, a largely Sunni city in Shiite-majority Iraq.

Using a derogatory term for Shiites, he says followers of the Muslim sect want to drive "empty Iraq of Sunnis and replace them with the worst of people."

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

The audio message could not be independently verified but it was similar to recordings previously released by al-Baghdadi.

The Islamic State group is fighting to hold Iraq's second city of Mosul as Iraq forces and allied Kurdish forces advance on the city with U.S.-led coalition support.

The city of more than one million people and surrounding territory fell to IS 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.