Islamic State militants are losing access to the sources of revenue that once gave them their power. That's prompting the extremist fighters to turn to extortion, kidnapping or foreign donations like their predecessors, al-Qaida.

The change in tactics is taking place as the Islamic State group sees its territory shrink and its dreams of an established caliphate evaporate. And it comes as the battle to retake the city of Mosul gets underway.

The Treasury Department's assistant secretary for terrorist financing, Daniel Glaser, says IS militants are being denied access to revenue sources such as oil and gas and cash reserves that amounted to more than $1 billion in 2014.