A view of Baiji oil refinery, north of Baghdad, which ISIS took a hold of in 2015 but was later re-captured by Iraqi forces.

U.S.-led coalition air strikes against Islamic State of Iraq and Syria (ISIS) oil facilities have cut the militant group’s oil revenues by about 30 percent since October, a coalition spokesman said on Wednesday

ISIS, an adversary the United States calls the wealthiest terrorist group of its kind in history, derives funds from a variety of sources, including sales of oil produced in territory it controls, extortion, and sales of antiquities, experts say.

In October, the U.S. military launched an intensified effort to go after oil infrastructure controlled by ISIS, dubbed “Tidal Wave II,” named after the bombing campaign targeting Romanian oil fields in World War Two.

That effort has consisted so far of 65 air strikes, a Baghdad-based spokesman for the U.S.-led campaign, U.S. Army Colonel Steve Warren, said in a news briefing. Those strikes have cut IS oil revenue by about 30 percent, and cut their production from 45,000 barrels of oil per day to 34,000 bpd, Warren said.

“In addition to chipping away at their so-called caliphate, killing their leaders, we’re also hitting them in the pocketbook,” Warren said.

Defense officials estimate the group was earning about $47 million per month from oil sales prior to October.

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