The Islamic State group has looted up to $1 billion from banks in Syria and Iraq, contributing to the group's military endeavors as it has conquered large swaths of Syria and Iraq over the last year, a senior U.S. Treasury official Adam Szubin said on Thursday. The internal source of revenue has made it difficult for the U.S. to target ISIS' finances. 

"It has looted between $500 million and $1 billion from bank vaults captured in Iraq and Syria," Szubin said during a speech at Chatham House in London, according to NDTV. "And it has extorted many millions more from the populations under its control, often through brutal means."

The U.S. has worked with the Iraqi government to shut down some 90 banks operating in territory controlled by ISIS. While the buildings are still standing and some of the banks may be operating domestically, they have been cut off from global financial systems, CNN reported.

ISIS has also brought in more than $500 million in trading oil, the treasury official said. The group’s major client has been the government of Syrian dictator Bashar Assad, despite the group being an enemy of the regime, Szubin said.

"The two are trying to slaughter each other and they are still engaged in millions and millions of dollars of trade," Szubin said, commenting on trade between the Syrian regime and ISIS.

A U.S.-led military campaign has been bombing ISIS targets for more than a year, aiding forces on the ground in retaking significant territory from ISIS in recent months. The anti-ISIS efforts have increasingly focused on targeting revenue sources, in particular oil infrastructure, but the group continues to bring in millions of dollars each month through confiscation of property and looting. An ISIS finance chief was recently killed in an attack, U.S. officials announced on Thursday.

There were some signs that the group had been struggling financially due to the anti-ISIS campaign. The militant group reportedly cut fighters’ salaries, increased the cost of electricity and other goods and introduced new taxes, a report by the London-based analysis firm IHS said earlier this week. ISIS maintains control over a number of major cities in both Iraq and Syria and has recently carried out attacks outside Syria and Iraq.