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Tens of millions of dollars in ISIS cash went up in smoke when one of its banks was hit by an American airstrike, U.S. defense and military officials told NBC News on Monday.

The head-chopping terrorists lost the fortune in an attack that was launched just before dawn Sunday on the ISIS-held Iraqi city of Mosul, the officials said.

They would neither confirm nor deny reports out of Iraq that innocent civilians were killed or that there guards in the bank at the time of the raid.

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But since November, U.S. and coalition forces have been trying to kill ISIS by going after its main money source — oil smuggling. They have been bombing ISIS-held oil fields and targeting tanker trucks carrying the black gold to Turkey.

In July, NBC News reported that the Islamic fanatics were making three times as much from oil smuggling as U.S. officials previously thought, roughly $8 million to $10 million a month.

Related: ISIS Makes Three Times as Much from Oil Smuggling as Previously Thought: Officials

The new intelligence about ISIS finances was discovered after U.S. commandos killed the militants' "money man, known as Abu Sayyaf, last May in a raid on his Syrian hideout. They also captured his Iraqi wife, identified as Umm Sayyaf, and seized several computers from their compound.

ISIS uses some of the blood money to pay its fighters monthly salaries and provide stipends to their families.

Foreign fighters, who are the highest-paid recruits in the ISIS ranks, earn as much as a $1,000 a month, two Syrian sources said.

Also, unlike al-Qaeda, ISIS is running a state in the Syrian and Iraqi territory it has taken over and needs money to keep the schools running and to fund a complex welfare system that enables it to buy the loyalty of its subjects.