BP\'s tumultuous stay in Russia was reportedly set for an extension on Thursday after a government source said the British firm was in \"serious\" talks about acquiring a stake in the state oil giant Rosneft. The news sent Rosneft\'s stock up more than three percent and reaffirmed speculation that Russia\'s largest oil company was about to cement its control of the local market by taking BP\'s place in a troubled joint venture. Russia\'s First Deputy Prime Minister Igor Shuvalov had earlier said the state could cede control of up to 15 percent of Rosneft through either an exchange of shares or some other form of major assets with a world major. \"This issue is being discussed seriously by BP, Rosneft and the government,\" the unnamed cabinet official was quoted as saying by the state-run RIA Novosti news agency. The possibility of Rosneft taking over BP\'s half of its lucrative but troublesome TNK-BP venture has been rumoured since the British firm confirmed fielding offers on June 1. Rosneft and its new chief executive Igor Sechin -- a trusted colleague of President Vladimir Putin who tried striking an historic tie-up with BP last year -- have both denied making formal bids for the British stake. But an asset swap with BP fits into both company\'s strategies as they seek to repair relations following a year of acrimony. BP is keen to re-establish its foothold in Russia after pursuing a preliminary share-swap and joint Arctic oil exploration agreement that then ended up in the hands of ExxonMobil. The $16 billion deal would have taken BP\'s stake in Rosneft from the current 1.25 percent to 10.8 percent in exchange for five percent percent of its own shares. The alliance was torpedoed by successful court petitions from billionaire partners who control the Russian half of TNK-BP -- the latest of many disputes to have rocked the joint venture since its formation in 2003. TNK-BP provides the British group with more than a quarter of its oil output and billions of dollars in dividends. But BP investors also view TNK-BP as a drag on the stock\'s performance because of uncertainties about its Russian venture\'s future and possible state intervention in its affairs. Rosneft for its part is cautiously looking ahead to a partial privatisation under a government programme that would still see the state retain control in the immediate term. Sechin\'s company has made clear it was only looking for major international investors that could help Rosneft tap Arctic and other hard-to-access fields as well as Western financial markets. Putin himself said on Thursday that foreigners should be ready to offer Russia big returns if they wanted pieces of the state\'s most prized assets. \"We will unquestionably welcome the involvement of our foreign partners in privatisation deals,\" Putin told the annual Saint Petersburg Economic Forum. \"But these must really be serious, strategic investors who bring along modern technology, experience in organising production, and large export orders,\" Putin said. Shuvalov for his part told The Wall Street Journal that he would only want the state taking over the British half of TNK-BP if this saw BP expand its presence in Rosneft. \"I wouldn\'t want to see a state company buy it (BP\'s stake) except as part of a deal to bring in another foreign partner or as part of a larger transaction to privatise Rosneft,\" Shuvalov told the US business daily. Shuvalov added that he would prefer to see BP\'s stake taken over by the Russian partners -- an option that the Financial Times said on Wednesday was being discussed in London by TNK-BP co-owner Mikhail Fridman.