Sources tell CBS News that the Islamic State of Iraq and Syria (ISIS) is now bringing in $1 million every day in oil revenue from one Syrian province alone.
CBS News correspondent Wyatt Andrews reports that since ISIS -- also known as ISIL and which now calls itself simply the "Islamic State" -- expanded into northern Iraq, its earnings from selling oil and gas on the black market for cash are believed to total roughly $3 million per day.
Former U.S. intelligence analyst Lori Plotkin Boghardt says those energy profits give ISIS its edge.
"ISIS is worth hundreds of millions of dollars," Boghardt says, making it easily the wealthiest terrorist group ever.
Photos taken from an ISIS fighter's social media page claim to show some of that cash. One reads, "Drown in your envy, hypocrites," in Russian. "The Caliphate is getting richer!"
Plundering natural resources isn't ISIS' only method of criminal income. The group has looted buildings, extorted local businesses in its territory and levied taxes on civilians.
U.S. officials have focused on another revenue stream: private donations.
In a message posted online, one Saudi sheikh implores his audience to "do jihad with your money." And the donations he seeks are weapons-specific. For 100 Kuwaiti dinars, a donor can supply eight mortars, while 50 dinars buys 150 bullets.
The U.S. has complained, but the Gulf states have internal reasons for allowing the donations to continue.
"In Kuwait, some of terrorist fund-raisers are very powerful and popular, and the government is very concerned about upsetting this domestic constituency," Boghardt said. "The irony is that some of America's closest allies in the Gulf allow their citizens to support terrorist groups."
The U.S. has slapped sanctions on some Gulf citizens in recent months for funneling money to extremist groups in Syria.
But ISIS, by running a cash business and operating outside the banking system, is making it much harder for the U.S. to disrupt its unprecedented financing.