The Islamic State raked as much as $1 billion in cash when it took control of Mosul, Iraq's second largest city, in 2014, a top Obama administration official disclosed.

The Islamic extremists took the amount when they seized the reserves of more than 90 banks in Mosul in northern Iraq, Daniel Glaser, the US Treasury's assistant secretary for terror financing, told the Aspen Security Forum last week, Fox News wrote.

Mosul has been under ISIS control since June last year and is said to be the group's main stronghold in the Middle East.

Aside from the cash from banks, ISIS also takes in $40 million a month in oil revenues, he added.

The flow of cash in ISIS' coffers allows the group to pay each of its fighters $1,000 per month.

The ISIS way of financing its operations was revealed following a May raid which killed Abu Sayyaf, who headed the group's oil and gas operations in eastern Syria.

The raid, Glaser said, yielded a "treasure trove" of information which allowed outsiders to understand the huge financial operation that supports the terror group's activities.

The US Treasury official's revelations on ISIS terror financing were released on the same day Senator John McCain, R-Arizona, had a snappish exchange with Marine Corps Lt. Gen. Bob Neller, who maintained that the Iraqi authorities are the ones who should win back Mosul for themselves.

In a hearing for his confirmation for the post of Marine Corps Commandant before the Senate Armed Services Committee, Neller also said he did not believe that ISIS is gaining in Iraq.

McCain then told Neller angrily: "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," added the senator.

"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."

The US government is still carrying out airstrikes against ISIS in Iraq, the US Department of Defence said on Saturday.

The coalition's aerial assaults on Friday totalled 22, seven of which were launched near the Iraqi city of Fallujah. The strikes hit a vehicle-borne improvised explosive device facility, a weapons repair facility and five ISIS bridges.

The coalition also carried out five airstrikes near Ramadi, which struck three ISIS tactical units and destroyed three of the group's fighting positions.

Other airstrikes were launched in Mosul, Al Huwayjah, Bayji, Habbaniyah, Makhmur, Sinjar, and Tal Afar.