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Military intelligence believes ISIS leader Abu Bakr al-Baghdadi abandoned Mosul prior to the battle to isolate the Iraqi city five months ago, a senior defense official confirmed.

The official said that they think Baghdadi, who called himself the ruler of all Muslims after taking Iraq’s second-largest city in 2014, is still alive but added that killing him is not a priority for the success of Operation Inherent Resolve — the U.S. military’s operation to combat ISIS.

Reuters reported that Baghdadi left Mosul’s command to a group of die-hard followers and is likely hiding out in the desert with sympathetic civilians, focusing on survival. They add that it seems he and ISIS have essentially ceded the city, which is the largest population center they have held.

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The terror group has approximately 15,000 fighters between Iraq and Syria, and currently maintains 2,500 in West Mosul and Tal Afar, where most of the fighting is now taking place, the official said. There are thousands more in the Euphrates River Valley, the official added.

Despite those numbers, the amount of willing fighters is beginning to drop dramatically, military intelligence confirmed.

Related: Kabul Hospital Attack: ISIS Claims Responsibility After 30 Killed by Gunmen Disguised as Doctors

Meanwhile, ISIS maintains a powerful presence in Raqqa, Syria, with 3,000 to 4,000 combatants, but some of their leadership has already fled that ravaged city in anticipation of a future assault, according to the defense official. Senior ISIS figures may have concluded that holding Raqqa is no longer "tenable," the official added.

Nevertheless, they leave Raqqa heavily fortified with trenches, tunnels and house-born IEDs. It will be a “long, hard, deliberate battle” for multi-national forces attempting to free the region from ISIS, the defense official said.

Raqqa is not expected to be the final or decisive battle in the ongoing conflict to end ISIS’s reign — but it will be critical for the region.

Leadership of Operation Inherent Resolve has yet to decide whether to chase ISIS forces into the Euphrates River Valley, where Russian forces have been lying in wait.