Several leaders of the Islamic State group have been killed in US airstrikes in northern Iraq in recent weeks, a US defense official said Thursday.

"These were the result of a series of airstrikes this month carried out over the course of several days," the official told AFP on condition of anonymity.

"We've long said that command and control, including leadership, remain valid targets."

The strikes were "part of degrading ISIL's ability to conduct command and control" of their forces, the official said, using an alternative acronym for the group.

The official said the operation was "not insignificant," and reflected a wider effort to pile pressure on the group as Iraqi forces prepare for a major counteroffensive in the coming months.

The jihadist leaders who were killed did not include the chief of the extremist group, Abu Bakr al-Baghdadi, the official added.

The Wall Street Journal first reported that US forces had taken out several key leaders, quoting the military's top officer, Gen. Martin Dempsey.

"These are high-value targets, senior leadership," Dempsey told the Journal.

The newspaper, quoting unnamed officials, said between Dec. 3-9, air raids killed a man known as Abd al-Basit, the head of the group's military operations in Iraq, and Haji Mutazz, a deputy to Baghdadi, the IS chief.

A third figure, Radwin Talib, the IS group's leader overseeing the city of Mosul, was killed in a strike in late November, the Journal reported.

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