In Syria, George Haswani sees himself as a patriot. In the West, he is a wanted man.

Mr. Haswani acts as a middleman between Islamic State and the Syrian government, the terror group’s largest customer, Western security officials allege. Islamic State controls much of Syria’s energy infrastructure and sells stolen oil and natural gas at a discount—even to the regime it is ostensibly battling.

Emerging from the fog of Syria’s multisided civil war, the businessman, 69 years old, says he is helping keep his country from plunging into the dark ages, given that Syria’s power plants run on fuel controlled largely by Islamic State. To the Syrian nuns he helped free from extremist kidnappers, Mr. Haswani is a hero.

U.S. and European Union officials, meanwhile, have sanctioned Mr. Haswani, a dual-Russian-Syrian national, for his alleged role as a broker of crude-oil shipments from Islamic State to the government of President Bashar al-Assad.The sanctions freeze assets held by Mr. Haswani in the U.S. and EU.