ISIS's finance minister was killed in air strikes alongside two other senior leaders in recent weeks, a US military spokesman said yesterday.

Army Colonel Steve Warren told a Pentagon briefing that US-led 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.

"Killing him and his predecessors exhausts the knowledge and talent needed to coordinate funding within the organisation."

The announcements comes as a US treasury official said ISIS had made more than £330m ($500m) trading oil.

ISIS' "primary customer" has been Syria's President Bashar al-Assad, despite their ongoing battle to overthrow his government's regime, according to the BBC. The terrorist group has also looted up to $1bn from banks in its territory, said US treasury official Adam Szudin.

ISIS, which is US says is the wealthiest group of its kind in history, is estimated to make as much as $40m every month from the oil trade. As well as Assad's government, Turkey were another major customer.

Szubin explained that ISIS did not rely on funding from foreign donors like other terrorist groups but generated money from its own operations.

Cutting off the group's cash flow is one of the main aims of America's air strikes which have focused on targeting ISIS-held facilities for over a year. The Pentagon has recently begun what it calls "Tidal Wave II", redoubling efforts to hit refineries, tankers and oil fields.