Moscow ? An American oil executive on Tuesday was named as the new chief executive officer of Yukos, Russia’s biggest and most embattled company.

Simon Kukes, 56, who was born in Russia but emigrated to the United States 30 years ago, replaces Mikhail Khodorkovsky, the former CEO who announced his resignation Monday from his Moscow prison cell.

Kukes received a doctorate from the Academy of Sciences of the USSR, then left the country and did post-doctoral work at Rice University in Houston. He joins two other U.S. executives atop the Yukos leadership — Steven Theede, of Hutchison, Kan., executive director of the Yukos-Moscow division, and Bruce Misamore, of Houston, the chief financial officer.

Khodorkovsky, 40, Russia’s wealthiest man with a personal fortune approaching $10 billion, was arrested two weeks ago in Siberia in a predawn raid by masked federal agents. He’s facing seven criminal charges, including fraud and tax evasion. The government has frozen his shares in Yukos as a hedge against possible fines and back taxes.

Many analysts say his arrest was politically motivated. He has clashed with President Vladimir Putin and has been openly financing two opposition political parties — a brash move with parliamentary elections just a month away.

He also alarmed Russian nationalists by negotiating with ExxonMobil and ChevronTexaco to sell off a sizable portion of Yukos.

The company makes up an estimated 5 percent of Russia’s gross domestic product and controls about a quarter of its oil.

From left, Bruce Misamore, chief financial officer of Yukos; Simon Kukes, Yukos new chief executive; Roman Artemyev, Yukos spokesman; and Steven Theede, executive director of Yukos-Moscow, prepare for a news conference in Moscow. Yukos officials said Tuesday that they had appointed Kukes as new chief executive. He replaces Mikhail Khodorkovsky, who has been jailed on charges of fraud and tax evasion.