Washington (CNN) The U.S. estimates that ISIS brought in $1 billion in revenue last year but that recent airstrikes on their cash holdings and oil infrastructure are beginning to squeeze the group's finances.

Daniel Glaser, the Treasury Department's assistant secretary for terrorist financing, said Thursday that the U.S. efforts to cripple ISIS' ability to raise money are "bearing fruit."

He pointed to airstrikes by the U.S.-led coalition against ISIS cash storage sites in Iraq and Syria as having destroyed "possibly" more than a $100 million of the group's money, along with strikes against oil infrastructure and distribution capabilities controlled by ISIS.

He said a combination of military power, intelligence sharing and diplomatic efforts are blunting ISIS' financial reach -- but that it still has paths to revenue at its disposal.

"ISIL's sources of income include oil and gas sales, extortion and taxation, external donations, kidnapping-for-ransom, and previously, bank looting," Glaser, using another term for ISIS, said in testimony before a joint meeting of the House Foreign Relations and Armed Services Committees.

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