The move boosts BP's equity investment in the country since the start of 2003 to around $US7.75 billion.

Last August, BP committed $6.35 billion in cash and shares to wrap most of its existing Russian assets into a 50-50 joint venture with assets of Russian investment firms Alfa Group and Access Renova, owners of TNK, to create TNK-BP.

The move into Russia has been considered by some as a high-risk strategy since the arrest of the country's richest man and former chief of the oil giant Yukos, Mikhail Khodorkovsky. Khodorkovsky was arrested for fraud and tax charges although many observers believe it was a politically motivated move.

Since then a merger deal between Yukos and rival company Sibneft has collapsed and many western companies have become very wary about investing in the sector.

However, the Slavneft acquisition will add about 160,000 barrels a day to TNK-BP's output of 1.2 million a day. BP now holds a 25% stake in Slavneft through its 50% TNK-BP holding, although Slavneft will remain a separate company because the other 50% belongs to Sibneft.