Islamic State militants have been funding their global tyranny by trading black market oil for up to US$40 million (approximately AU$56.5 million) a month, the United States Treasury has revealed.

Copious volumes of oil have been sold to the government of Syrian President Bashar al-Assad, with small amounts also finding its way to Turkey.

As a result, the United States Treasury imposed sanctions on networks providing support to the Government of Syria, including for facilitating Syrian government oil purchases from the IS.

Assets designated to Syria, especially those including oil trade that are based in the United States or controlled by the nation, have since been frozen and transactions between the two countries prohibited.

Senior US Treasury official Adam Szubin defended the country’s actions, detailing how much oil the IS is selling to the Assad regime.

“The volumes we are talking about and the amounts of money we are talking about are very sizeable,” he said.

“The two are trying to slaughter each other and they are still engaged in millions and millions of dollars of trade,” he also said in reference to Assad’s government and IS.

It is estimated that the Daesh militants have made more than US$500 million (approximately AU$648 million) in funding from black market oil sales. While the majority of Daesh oil ends up under Assad’s control and is consumed internally, Szubin said some of the product also turns up in Kurdish regions and Turkey.

“Our sense is that the ISL is taking its profits basically at the wellhead,” he said.

“While you do have ISIL oil ending up in a variety of different places, that’s not really the pressure we want when it comes to stemming the flow of funding - it really comes down to taking down their infrastructure."

The United States has worked with Iraq to cut militant ties to the global banking system, Szubin said. Dozens of branches were closed down in Daesh-held territories after rebels had looted almost US$1 billion (approximately AU$1.3 billion) from bank vaults in Syria and Iraq.

Szubin said the Daesh oil trade is the US Treasury’s main target in cutting the Assad regime out of Syria.

“The United States will continue targeting the finances of all those enabling Assad to continue inflicting violence on the Syrian people."

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