ISIS netted between $500 million and $1 billion in cash when it took over Iraq’s second largest city in 2014, a top Obama administration official said Thursday.

Daniel Glaser, the U.S. Treasury’s assistant secretary for terror financing, told participants at the Aspen Security Forum that when ISIS swept through the city of Mosul in northern Iraq, it took the reserves of over 90 banks, estimated to be between $500 million and $1 billion.

The northern Iraqi city has been occupied by ISIS since June 2014 and is considered to be its main stronghold in the region.

Glaser also said that the May raid that killed Abu Sayyaf, a key ISIS leader in charge of the group’s oil and gas operations in eastern Syria, was a “treasure trove” of information on ISIS financing, giving new insight into the enormous financial operation propelling the terror group.

The Islamist group is also taking in $40 million a month in oil revenues alone, while ISIS fighters are paid $1000 a month, Glaser said.

Glaser’s revelations came on the same day as a testy exchange between Sen. John McCain, R-Ariz., and Marine Corps Lt. Gen. Bob Neller at Neller’s confirmation hearing for the post of Marine Corps. Commandant before the Senate Armed Services Committee.

When asked by Sen. Kelly Ayotte, R-N.H., if more could be done against ISIS in Iraq, Neller replied, “I think we’re doing what we need to do right now,” adding that it was up to the Iraqis to retake their lost territory from ISIS, and later saying that he didn’t believe ISIS was winning in Iraq.

McCain replied angrily to Neller, saying, “You know, I don’t know where you’ve been. Obviously ISIS is winning in Iraq.”

“And this line about they’re the ones that have to do it themselves…General, they can’t do it themselves. We know that. The Iraqis cannot do it themselves. That’s why they’ve lost their second largest city,” McCain said.

“And for you to say we’re doing what we need to do, then maybe you can tell me what we’re doing that will win against ISIS,” McCain added.

Fox News’ Catherine Herridge and Lucas Tomlinson contributed to this report.