WASHINGTON (MarketWatch) - The Islamic State has made more than $500 million from black market oil sales, according to Adam Szubin, Treasury's acting secretary for terrorism and financial intelligence. In a speech in London, Szubin said the group was "wealthy," in part because it has looted up to $1 billion from bank vaults captured in Iraq and Syria and extorted many millions more from the population under its control. It is difficult for the U.S. to constrain the group's funding because only a small share of its funding comes from overseas donors, he said. Szubin said ISIS "has its vulnerabilities because "waging a multi-front war while attempting to essentially act as a proxy-state requires steady and renewable sources of funding."