The Islamic State has acquired about $1.5 billion dollars through black market oil sales and looting bank vaults, according to an official within the United States Treasury Department.

Acting Under Secretary for Terrorism and Financial Intelligence Adam Szubin spoke at the Royal Institute of International Affairs in London on Thursday, providing insight into the Islamic State’s bankroll. In remarks prepared for delivery, Szubin said black market oil sales have netted the terrorist organization more than $500 million and militants have also looted between $500 million and $1 billion after seizing various bank vaults throughout Iraq and Syria.

The group has also extorted millions more from those living under its control, Szubin said. His prepared comments did not mention a specific window of time in which ISIS acquired the funds.

Reuters reported that Szubin said the Islamic State was “selling a great deal of oil” to the regime of Syrian President Bashar al-Assad, an astonishing revelation given that the Islamic State is currently entrenched in a war with Syrian government forces. Islamic State oil was also being shipped to Turkey and some Kurdish regions in the Middle East, though Reuters quoted Szubin as saying that a “far greater amount” winds up in the hands of al-Assad and his government.

Russian officials had previously accused Turkish president Tayyip Erdogan and his family members of personally benefiting from the Islamic State’s black market oil trade, though Erdogan has publicly denied that and said he would resign if the allegations were proven true.
The two countries have been at odds since Turkey shot down a Russian warplane near the Syrian border last month, with each side believing a different set of the circumstances about the event.

United States officials have called for Turkey to seal its border with Syria to disrupt the flow of oil and manpower into Islamic State strongholds, one of several tactics designed to combat ISIS.

The United States is leading a 65-nation coalition, which Britain recently joined, that is carrying out airstrikes against Islamic State interests in Syria. Those airstrikes have been known to target Islamic State-controlled oil fields, with the ultimate goal of choking off the group’s fund supply.

In his remarks prepared for delivery, Szubin said United States officials are also taking steps to prevent the Islamic State from spreading its money internationally. The U.S. is working with a at least 30 countries to prevent the group from performing tasks like wire transfers. The U.S. has also sanctioned at least 30 leaders and financiers of the Islamic State in 2015 alone, Szubin said.