LONDON — ISIS militants have made more than $500 million trading oil with significant volumes sold to the government of Syrian President Bashar Assad and some finding its way to Turkey, a senior Treasury official said on Thursday.

A U.S.-led coalition is bombing the hardline Sunni fighters, as is Assad's only big-power supporter Russia, in an attempt to kill ISIS leaders and cripple the oil wells which the group uses to finance its rule and attacks abroad.

In one of the most detailed public explanations of ISIS' oil trade, Treasury Department official Adam Szubin said militants were selling as much as $40 million a month of oil at the installations which was then spirited on trucks across the battlelines of the Syrian civil war and sometimes further.

"[ISIS] is selling a great deal of oil to the Assad regime," Szubin, acting under secretary for Terrorism and Financial Intelligence with the Treasury, told an audience at 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 said of Assad's government and ISIS.

The "far greater amount" of ISIS oil ends up under Assad's control while some is consumed internally in areas controlled by the group. Some ends up in Kurdish regions and some in Turkey, he said.

"Some is coming across the border into Turkey," Szubin said when asked for details on the money trail.

Szubin said it was unclear whether the $40 million a month estimate could be multiplied over a year. But in remarks prepared for delivery, he said ISIS had made more than $500 million from the oil trade, but did not give a more specific time period.

After Turkey downed a Russian fighter jet last month, Russian President Vladimir Putin said he had intelligence that large amounts of oil and petroleum products were moving across the border from ISIS territories to Turkey.

The son of Turkish President Tayyip Erdogan has denied Russian allegations that he and his family were profiting from the illegal smuggling of oil.

"There is no question that better security, closing the Turkish border to flows is a key component right now and we are looking to the Turks to do more in that respect," Szubin said.

In an attempt to cut militants' links to the global financial system, Szubin said the United States had worked with Iraq to close down dozens of bank branches in ISIS-held territories. According to Szubin, the militants had looted up to $1 billion from bank vaults in Syria and Iraq.