The Associated Press reports that "the Islamic State rakes in up to $50 million a month from selling crude from oil fields under its control in Iraq and Syria, part of a well-run industry that U.S. diplomacy and airstrikes have so far failed to shut down, according to Iraqi intelligence and U.S. officials."
"Oil sales — the extremists' largest single source of continual income — are a key reason they have been able to maintain their rule over their self-declared 'caliphate' stretching across large parts of Syria and Iraq. With the funds to rebuild infrastructure and provide the largesse that shore up its fighters' loyalty, it has been able to withstand ground fighting against its opponents and more than a year of bombardment in the U.S.-led air campaign."
"The group has even been able to bring in equipment and technical experts from abroad to keep the industry running, and the United States has recently stepped up efforts to close off this support."