"We are systematically eliminating ISIL's cabinet," Carter told reporters, referring also to the killing early in the month of Omar al-Shishani, the man known as "Omar the Chechen," who was effectively IS's defense minister.
The latest killing "will hamper the ability for them to conduct operations inside and outside of Iraq and Syria," Carter said of Abd ar-Rahman Mustafa al-Qaduli, referring to him as Haji Imam. He said al-Qaduli served as the group's finance minister and had been behind some foreign plots.
Carter declined to say whether al-Qaduli had been killed by a drone strike or in a bombing raid involving manned aircraft; nor would he specify whether the attack occurred in Syria or Iraq, though he said that any action in Iraq would only have been taken with Iraqi government approval.
The US Justice Department had offered a bounty of up to $7 million for information leading to al-Qaduli.
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"A few months ago when I said we were going to go after ISIL's financial infrastructure, we started with storage sites, and now we've taken out the leader who oversees their finances, hurting their ability to pay and hire recruits," Carter said.
But whether Islamic State group militants in Iraq and Syria like al-Qaduli had specifically directed or simply inspired such attacks, the US-led forces would do their best to eliminate them, Carter said.
"Our campaign plan is first and foremost to collapse ISIL's parent tumor in Iraq and Syria, focusing on the power centers in Raqqa and Mosul," he said.
Carter said local forces supported by the US-led coalition had "severed the main artery between Syria and northern Iraq," making it "much harder for ISIL's leaders and forces to travel between Raqa and Mosul."