The United States is spending between $7 million and $10 million each day in the battle against ISIS, and the Pentagon said Friday it needs more money from Congress.

"We're generally spending roughly, since this effort started, $7 million to $10 million a day. That's being funded out of OCO, overseas contingency operations," Defense Secretary Chuck Hagel said at a Friday press conference. Commonly referred to as war funds, the money isn't subject to congressional budget caps.

The Pentagon estimated in late August that it was spending $7.5 million a day in Iraq, where it has conducted more than 200 airstrikes. Since Monday, the U.S. and its allies have launched more than 40 strikes against targets in Syria.

Asked Thursday how much the U.S. was spending, Rear Adm. John Kirby, a Pentagon spokesman, said the department is still working to calculate official costs. He added that he "wouldn't be surprised if the answer that we come back with once we do the pencil work is different" than any estimates.

Defense Department officials are also working to project what the long-term costs will be for the ISIS fight, which is expected to take years.

Whatever the official budget ends up being, Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said they'll have to go back to Congress to ask for more money.

"We're working now with appropriate committees on how we go forward with authorizations and funding," Hagel said.

Congress isn't expected back in Washington until after the midterm elections in November. Lawmakers from both parties have suggested that the ongoing conflict could provide the needed push to undo sequestration.

Dempsey rejected the notion that OCO should be able to cover anti-ISIS operations. He tied the funding of the ISIS operation to the larger financial crunch the department is facing as it prepares its fiscal 2016 budget.

Without action from Congress, the budget caps, known as sequestration, would return in October 2015, the start of the 2016 fiscal year. Under President Obama's five-year budget, the Pentagon projects that it will need more than $535 billion in fiscal 2016. But under the budget caps, the Pentagon is expected to receive less than $500 billion, leaving a $35 billion budget gap.

"If you are asking me do I assess right now as we go into the fall review for '16 that we're going to have budget problems? Yes," Dempsey said. "... OCO is gas money. The baseline budget is what trains and equips and organizes a force."