WASHINGTON (Reuters) - The U.S.-led coalition fighting Islamic State militants in Iraq and Syria has killed the group’s finance minister and two other senior leaders in air strikes in recent weeks, a U.S. military spokesman said on Thursday.

Army Colonel Steve Warren told a Pentagon briefing coalition strikes had killed Abu Salah, Islamic State’s financial minister, in late November.

“He was one of the most senior and experienced members of ISIL’s financial network and he was a legacy Al Qaeda member,” Warren said.

Warren said Salah was the third member of the finance network who had been killed in as many months.

“Killing him and his predecessors exhausts the knowledge and talent needed to coordinate funding within the organization,” Warren added.

According to a Brooking Institute report, Abu Salah’s real name is Muafaq Mustafa Mohammed al- Karmoush.

Warren said a senior leader responsible for coordinating the group’s extortion activities and another leader who acted as an executive officer had also been killed.

The Islamic State, which the United States calls the wealthiest militant group of its kind in history, has a number of revenue streams.

It has built up what amounts to a “durable and resilient financial portfolio,” funded by oil sales, extortion, and sales of antiquities, said Thomas Sanderson, an expert on terrorism at the Center for Strategic and International Studies.

Recently, U.S. defence officials estimated that Islamic State was earning about $47 million per month from oil sales prior to October.

That same month, the Pentagon started an operation known as “Tidal Wave II” targeting Islamic State’s oil revenue.

Warren said it was too early to tell what the impact of the strikes had been and that it was similar to a boxing match.

“You work the body in (the) early rounds and maybe you won’t get a knockout from those early round body shots, but several rounds later you’ll see your... opponents knees begin to get weak ... and now he’s beginning to get set up for the knockout,” Warren said.