ISIS has become the world’s wealthiest terror group, earning tens of millions of dollars a month from illegal oil sales and ransoms, officials said Thursday.

“We have no silver bullet, no secret weapon to empty ISIS’ coffers overnight. This will be a sustained fight, and we are in the early stages,” said undersecretary for terrorism and financial intelligence David Cohen.

Cohen is among a team of Obama administration officials leading the fight against ISIS, which has seized a large swath of territory in Iraq and Syria. […]