MOSCOW (Reuters) - Oil major BP <BP.L> said on Saturday it is suing its partners in the Russian venture TNK-BP <TNBPI.RTS> in London for 8.4 billion roubles ($357.3 million), money it believes should not have been paid out as back-tax claims in Russia.

The move comes on top of other problems at TNK-BP, whose owners, BP and four Russian-connected billionaires, are locked in a dispute over the firm's strategy and management control.

"It relates to our initial shareholding agreement signed back in 2003 which said that all back-tax claims prior to the creation of the TNK-BP venture must be paid by the Russians only," BP spokesman in Moscow Vladimir Buyanov told Reuters.

"It has nothing to do with the current dispute," he added.

TNK-BP has had to pay over $2 billion in back-tax claims over the past few years, with some claims relating to the period prior to 2003, before the venture was set up.

BP has already turned down a demand by the Russian shareholders for TNK-BP CEO Robert Dudley to resign and the Russian co-owners have said they would seek other legal means to limit BP's influence on the company.

Mounting state pressure on the firm in recent months, including back tax claims, office raids and the arrest of an employee on espionage charges, are signs that the Kremlin is putting pressure on either BP or the Russian billionaires to sell out to a state-controlled company, analysts say.

(Reporting by Dmitry Zhdannikov, writing by Amie Ferris-Rotman, Editing by Peter Blackburn)