A senior U.S. treasury official revealed that Syrian dictator Bashar al-Assad has been the biggest buyer of oil from ISIS, with the terror group making over $500 million from oil sales to his regime and others, Reuters reported on Thursday.

“[ISIS] is selling a great deal of oil to the Assad regime,” Adam Szubin, the acting under secretary for Terrorism and Financial Intelligence with the Treasury Department, said in remarks at the Chatham House in London. “The two are trying to slaughter each other and they are still engaged in millions and millions of dollars of trade.”

Szubin explained that after drilling the oil, ISIS transports it in tankers westward into Syria, where a significant amount of it ends up in Assad’s territory. He added that some quantities are also sold in Turkey and Kurdish areas, though a “far greater amount” is bought by the Syrian regime. According to his estimate, ISIS is selling as much as $40 million worth of the commodity every month.

In November, the Treasury Department designated six entities and four people for their support of Assad, including one who acted as a middleman for Syria’s oil purchases from ISIS.

Days earlier, Secretary of State John Kerry described the “symbiotic” relationship between Assad and ISIS, saying, “They sell oil. He buys oil.” Kerry similarly emphasized the connection between the regime and the terror group in December, noting, “It was Assad’s ruthless reign that fueled Daesh’s rise and enabled terrorists to portray themselves as the only alternative Syrians had to their dictator.”

David Blair, the chief foreign correspondent of The Telegraph, outlined earlier this week how Assad deliberately released jihadists from his jails in 2011 and 2012, some of whom went on to become ISIS commanders. Josh Rogin of Bloomberg View explained last month that Assad’s strategy of bolstering ISIS to create a false choice between him and the terror group was working, and noted that the attention ISIS was receiving in the wake of the terror attacks in Paris was “taking the pressure off the Syrian regime right at the moment when pressure might have been effective.”

In June, the administration charged Syria with bombing non-Islamist rebels while ignoring ISIS targets, which allowed the terror group to advance on the city of Aleppo in northern Syria. A month earlier, former U.S. military intelligence officer Michael Pregent explained that Iran, Assad’s chief sponsor, and its allied militias did not extend themselves to fight ISIS, and concluded that “Iran needs the threat of ISIS and Sunni jihadist groups to stay in Syria and Iraq in order to become further entrenched in Damascus and Baghdad.”

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