An airstrike by the international coalition led by the United States has destroyed a cash storage facility used by Islamic State (ISIS) jihadists in the Iraqi city of Mosul, an American defense official said Monday.

Two 2,000-pound (900-kilogram) bombs struck the facility, destroying “millions” of dollars worth of cash, the official told AFP, speaking on condition of anonymity.

“We estimate in the millions of dollars… from all their illicit stuff: oil, looting, extortion,” the official told the news agency.

The strike reportedly came early Monday.

CNN, which first reported the strike, said the U.S. military believed between five and seven civilians had been killed.

ISIS has been described as the richest terrorist organization in the world and has even been the target of an economic campaign which aims to expose the inner economic workings of ISIS and related groups and to stop its campaigning through social media networks.

The American-led coalition carrying out plane and drone strikes against ISIS in Iraq and Syria has been increasingly targeting the jihadists’ money-making capabilities, including by bombing trucks that ferry illicit oil across Syria.

Under pressure from critics who say the campaign is moving too slowly, the Pentagon has indicated it would consider a wider array of targets even if these might cause civilian deaths, provided these attacks yield significant gains against the jihadists.

The defense official told AFP the coalition had targeted cash-holding facilities once or twice in the past year, but the most recent action was “probably” the biggest to date.

It was not immediately clear if the money had been in U.S. dollars, some other foreign currency, or local dinars, the official added.

AFP contributed to this report.