When Vladimir Putin brought Igor Sechin out of the shadows and into the Kremlin 14 years ago, Russian newspapers said they had no photographs of him and alluded to his behind-the-scenes influence by calling him Darth Vadar.

A man with no energy industry background, Sechin has served for two years as boss of Rosneft, a state-controlled company that under Putin swallowed up Russian oil assets to become the biggest publicly listed oil producer in the world.

A close lieutenant of Putin's for more than two decades, Sechin is the most important figure among a list of Russians whose U.S. assets were frozen on Monday as part of sanctions intended to push Moscow to stop supporting a pro-Russian uprising in eastern Ukraine.