Mosul is the largest city in a self-declared ISIS caliphate straddling the border between northern Iraq and eastern Syria.

The United States and Iraq have started preparing for an offensive by summer to recapture the Iraqi city of Mosul, which was taken by militants belonging to the Islamic State of Iraq and Syria (ISIS) last June, the Wall Street Journal reported on Thursday.

Preparations include selecting and training military units for the planned assault and cutting supply lines to ISIS fighters, Gen. Lloyd Austin, head of the U.S. military’s Central Command, told the Journal.

Mosul is the largest city in a self-declared ISIS caliphate straddling the border between northern Iraq and eastern Syria.

A senior Iraqi official told Reuters in November that Mosul was the focus of government efforts to defeat ISIS, because of the city’s size and symbolic status.

The U.S.-led forces have been conducting air strikes against ISIS targets in Iraq and Syria, including 16 since Wednesday that were centered on Mosul.

The Journal quoted Austin as saying that Iraqi forces must be ready before the offensive could begin.

“If we did things alone or with some of the other allies on the ground, it could move faster,” he told the newspaper. “But the Iraqis have to do this themselves.”

Austin said he had not yet determined whether to recommend that U.S. ground troops accompany local units pushing into Mosul but added the military would “do what it takes,” the Journal said.

He told the paper that two Iraqi divisions were expected to lead the force fighting to retake Mosul. The newspaper quoted military officials as saying those forces would receive four to six weeks of U.S. training.

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