Speaking at the nonpartisan Carnegie Endowment for International Peace, Cohen said Islamic State raises tens of millions of dollars a month through the sale of stolen oil, ransoms for kidnapping victims, theft and extortion in areas it controls and, to a lesser extent, donations from donors and financiers in Persian Gulf nations.

Although precise figures are impossible to obtain, U.S. investigators estimated that Islamic State earned about $1 million a day from oil sales beginning in mid-June, when the group’s military blitz through western Iraq vastly expanded its territory and its access to oil fields and refineries.

Coalition airstrikes and other efforts since August have “begun to impair” those sales, Cohen said. The International Energy Agency reported last week that the militant group’s ability to produce, refine and smuggle oil had been “significantly hampered.”