LONDON (CNN) - A representative of embattled Russian oil company Yukos said Wednesday the company has launched bankruptcy proceedings under U.S. law in a Houston court.

Claire Davidson, a spokeswoman for the company in London, did not indicate when the company filed for bankruptcy.

Yukos faces more than $18 billion in tax claims from the Russian government, of which it has paid off approximately $4 billion.

Former CEO Mikhail Khodorkovsky, a critic of Russian President Vladimir Putin, remains in jail as his trial on charges of fraud and tax evasion grinds on.

Once Russia's largest oil producer, pumping about 20 percent of the country's estimated 9 million barrels per day, Yukos has slipped to number two behind Lukoil.

Khodorkovsky funded opposition political parties and expressed a desire to run for office some day. Putin has said the case is part of a crackdown on corruption and denies any political motivation behind the Yukos probe.