The Treasury Department says Islamic State militants are earning about $1 million a month just from black market oil sales.

The official who leads the department's effort to undermine the group's financial strength says IS also has taken in at least $20 million in ransom payments this year from kidnappings.

David Cohen, the Treasury undersecretary for terrorism and financial intelligence, says the group takes in several million dollars a month from wealthy donors, and from extortion rackets and other criminal activities, such as robbing banks.

Cohen says that except for state-sponsored terrorist groups, the Islamic State group is probably the best-funded terrorist organization the Treasury Department has ever confronted.