December 11, 2015 10:52 AM EST

The Islamic State of Iraq and Greater Syria (ISIS) has made more than $500 million trading oil, according to a U.S. Treasury official.

The militants have sold much of the oil to Syrian President Bashar Assad’s government and some to Turkey, Reuters reports.

“The two are trying to slaughter each other and they are still engaged in millions and millions of dollars of trade,” Treasury Department official Adam Szubin said of the Syrian government and ISIS.

Szubin did not give a timeframe for the $500 million figure, but he said ISIS is making as much as $40 million per month selling oil.

Write to Tessa Berenson at tessa.berenson@time.com.

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