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The United States' military operations against ISIS militants in Iraq have cost an average of $7.5 million per day since they began in mid-June, the Pentagon said Friday.

That means the department would have spent about $500 million since it became directly involved in fighting Islamic State of Iraq and al-Sham terrorists.

"(The cost) has varied since the beginning in mid-June," said Rear Admiral John Kirby, Pentagon press secretary. "But on average it's costing about $7.5 million per day ... That's based on a snapshot of the operations that have occurred as of the 26th of this month."

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Kirby said the operations are being "supported through our overseas contingency funding. We're well within the limits that we need for 2014."

In 2007, when the U.S. was actively fighting different insurgents in Iraq, the military costs were estimated to be about $720 million a day. A Brown University study in 2013 estimated the total cost of the war to be at $2.2 trillion. That figure included care for veterans who were injured in the war — which researchers said could add up to $500 billion through 2053.

Meanwhile, U.S military forces conducted four more airstrikes in Iraq Friday, all centered around the Mosul Dam, which had been besieged by ISIS fighters. The strikes destroyed four ISIS armed vehicles, severely damaged another armed vehicle, and destroyed three ISIS support vehicles, the Pentagon said. All aircraft exited the strike areas safely.

Kirby said Friday that the dam is still under attack "almost every day" by ISIS.

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— Courtney Kube and Hasani Gittens