US-led coalition air strikes against Islamic State oil facilities have cut the militant group's oil revenues by about 30 percent since October, a coalition spokesman says.
Islamic State, 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 US military launched an intensified effort to go after oil infrastructure controlled by Islamic State, dubbed "Tidal Wave II", named after the bombing campaign targeting Romanian oil fields in World War II.
That effort has consisted so far of 65 air strikes, a Baghdad-based spokesman for the US-led campaign, U.S. Army Colonel Steve Warren, said in a news briefing on Wednesday (local time).
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, 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.
Defence officials estimate the group was earning about US$47 million (NZ$70 million) per month from oil sales prior to October.