* Oligarch co-owners have already expressed interest
* BP's stake in TNK-BP worth $25-$30 bln - analysts
* No deal possible for three months
* Final deal structure unclear, but state seen in control
* State banks would bankroll transaction - bankers
By Douglas Busvine and Megan Davies
MOSCOW, July 24 (Reuters) - State oil company Rosneft waded into the dispute between BP and its Russian partners on Tuesday, saying it is in talks to buy half of the troubled venture in a reminder the Kremlin is likely to fire the final shot in the battle.
British oil major BP has said it is seeking to exit its troubled but lucrative investment in Russia's third-largest oil company, TNK-BP. It formed the 50:50 joint venture with AAR, the consortium representing the tycoons, nearly a decade ago to tap into the country's vast energy reserves. The group is now estimated to be worth as much as $60 billion.
Rosneft's gambit signals the resolve of new Chief Executive Igor Sechin, the energy "tsar" in Russia's previous government, to build the country's largest oil company into a national champion of global scale.
"Rosneft has an established expertise and track record in managing large assets and considers the potential acquisition of BP's participation in TNK-BP to be an attractive commercial proposition," Sechin said in a statement.
"The potential acquisition would complement its existing portfolio and create value for all stakeholders."
AAR has already expressed its interest in buying half of BP's stake in TNK-BP, but would also consider selling for cash and BP stock to end a shareholder relationship that has broken down irretrievably.
BP said that while conducting discussions with Rosneft and any other interested parties, it will also negotiate in good faith with AAR.
Rosneft said it had agreed to enter negotiations with BP on buying its 50 percent stake in the joint venture and had signed a non-disclosure agreement.
AAR, led by banking-to-retail tycoon Mikhail Fridman, declined to comment. Last week AAR expressed its interest in buying 25 percent of BP's stake in TNK-BP, and has said it would be willing to pay $10 billion.
Under the TNK-BP shareholders' agreement BP cannot conclude a deal with anyone other than AAR for a period of 90 days.
BP's stake in TNK-BP , acquired for $7 billion in 2003 , is estimated by analysts to be worth between $25 billion and $30 billion.
"If this were to happen (Rosneft buys out BP), it would be the best-case outcome for BP, but the worst case for AAR and bad for (TNK-BP) minorities," Chris Weafer, chief strategist at Troika Dialog, said in comments emailed to Reuters.
BP, having agreed a deal with the state, would gain access to projects similar to those involving other Western energy majors, such as ExxonMobil, that have recently struck ambitious exploration agreements in Russia, Weafer added.
"It would still be in the Russia game, but with a cash payout and rid of its troublesome partners. A bit like getting a divorce and then marrying the judge."
Bankers and analysts said that, by selling, BP would realise a slug of cash that would help it to meet multibillion-dollar liabilities arising from the Gulf of Mexico disaster of 2010 but it would cut off much-needed cash flows from Russia.
BP has reaped $19 billion in dividends since buying into TNK-BP -- more than double its original investment. AAR has pointed out that TNK-BP contributed more than 90 percent to BP's dividends of $4.1 billion last year.
BP shares rose 0.4 percent by 1243 GMT, while Rosneft stock gained by 0.6 percent in Moscow. Shares in TNK-BP's listed unit, which has a small free float, fell by 0.6 percent.
Russian state-controlled banks would finance Rosneft's acquisition of BP's stake in TNK-BP, but would probably need the support of foreign lenders in a major syndicated deal, bankers said.
It is possible, analysts speculated, that Rosneft would buy all of TNK-BP in a deal that would create an 'oil Gazprom' with the sort of geopolitical clout that Russia's state-controlled gas export monopoly can project through its pipeline network.
Such a deal - involving a multi-billion-dollar payday for a group of unpopular business oligarchs - would present the Kremlin with a substantial public-relations challenge, however.
One banker close to AAR, meanwhile, said that its proposal to buy half of BP's stake was also bankable. Deal financing would be paid off within a few years out of TNK-BP's dividend flows, and the firm itself could be leveraged to finance a deal.
BP announced on June 1 that it would put its TNK-BP stake up for sale, giving AAR 45 days under the shareholder pact to express its interest.
Fridman quit as TNK-BP's chief executive in late May, days after Sechin was named Rosneft chairman in a move that signalled a major energy power shift in Russia after Vladimir Putin's re-election for a third presidential term.
Sechin had been thwarted a year earlier by AAR in his attempt to forge a strategic offshore exploration deal and share swap between Rosneft and BP. AAR, which had blocked the alliance in the courts, turned down a $32 billion last-ditch joint bid by Rosneft and BP to buy it out in May 2011.
The prospect of partnering now with a state company may not be palatable for Fridman and his co-investors -- German Khan, Viktor Vekselberg and Len Blavatnik. One scenario would be that it is they who end up exiting the business, some analysts say.
"My opinion is that the end deal is going to be AAR exiting rather than BP, because I really don't see a way for Sechin to work with AAR," said one analyst from a Western bank in Moscow, speaking on condition of anonymity.
"I think the end deal is BP and Rosneft together buying out the AAR stake, 25 percent each. That could easily be financed by Rosneft and BP."