Mikhail Khodorkovsky is seen behind bars in a Moscow court (2005 file photo)

Lawyers for imprisoned Russian oil tycoon Mikhail Khodorkovsky say authorities have filed new charges against their client.

The lawyers say they are still trying to determine the exact nature of the charges. But they indicate that those filed Monday appear to differ little from earlier charges against the imprisoned former oil executive. They called the new filing an effort to extend the inquiry into the case.

Khodorkovsky, the former chief of the Yukos Oil Company, is serving eight years in prison for fraud and tax evasion - charges he says were politically motivated because of his support for opposition politicians.

The former oil executive is being held in the Siberian city of Chita, where authorities have been investigating money laundering and theft charges against him.

The probe involves the theft of more than $33 million from Yukos subsidiaries. 

Some information for this report was provided by AFP and Reuters.