A senior US official revealed on Thursday that Islamic State (ISIS), the world’s richest terrorist organization, has made up to $1.5 billion from robbing banks in Syria and Iraq as well as black market oil sales – meaning it has higher revenues than many small countries.

“ISIL has made more than $500 million from black market oil sales. It has looted between $500 million and $1 billion from bank vaults captured in Iraq and Syria,” said senior US Treasury official Adam Szubin, using an alternate acronym for the terror group.

In addition to the oil sales and bank heists, ISIS “has extorted many millions more from the populations under its control, often through brutal means,” he said.

Speaking at Chatham House in London, Szubin was quoted by Al Jazeera as saying the US is working together with Iraq to prevent ISIS from accessing its funds.

Szubin noted that just in 2015, the US sanctioned over 30 “ISIL-linked senior leaders and financiers.”

“ISIL is wealthy. But it has its vulnerabilities. Waging a multi-front war while attempting to essentially act as a proxy-state requires steady and renewable sources of funding,” said Szubin.

“ISIL needs funds to pay fighters and procure weapons, to maintain its infrastructure, and to provide basic services to the people living in the territory it controls. And, ISIL needs to access the international financial system – to move money abroad, to transfer funds, and to import needed supplies.”

“We are targeting both of these dependencies – ISIL’s ability to generate revenue, and its ability to use that revenue.”

Szubin’s statements were made the same Thursday that the US announced it had killed ISIS’s finance chief Abu Saleh in airstrikes late last month.

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