The so-called Islamic State (IS) has made more than $500m (£330m) trading oil, a US treasury official has said.
Its "primary customer" has been the government of Syria's President Bashar al-Assad, despite its ongoing battle to overthrow the regime, Adam Szubin told the BBC.
IS had also looted up to $1bn from banks in territory it held, he said.
A US-led coalition has been bombing IS targets, including oil facilities, in Syria and Iraq for over a year.
IS' finance chief was recently killed in one such mission, Pentagon officials announced on Thursday.
"The two are trying to slaughter each other and they are still engaged in millions and millions of dollars of trade," Mr Szubin said of Syria and IS, in comments reported by Reuters news agency.
The group was estimated to be making as much as $40m a month from the oil trade, including from buyers in Turkey, he added.
Cutting off the group's cash flow was a key part of the coalition strategy to defeat IS, he said.
Unlike other designated terrorist groups, IS did not rely on funding from foreign donors, but generated money from its own operations, Mr Szubin said.
The US-led coalition has recently launched a military campaign, dubbed Tidal Wave 2, intensifying air strikes on IS oil fields, refineries and tankers being used by the group.
IS currently generates around $80m a month, mainly from oil revenues, according to findings focusing on late 2015 from UK defence consultancy IHS.
Other sources include taxation, drug and antiquities smuggling, robbery and kidnapping and the sale of electricity, it found.