One way or the other, Rosneft, the Russian state oil company, is poised to significantly expand its oil and gas holdings in Russia. And BP, its latest Western partner, could benefit from whatever move Rosneft makes.

Rosneft is reported to have concluded an agreement to buy the holdings of the four billionaires who own the country's third-largest oil company, TNK-BP.

BP, the other partner in TNK-BP, has also been in talks to sell its half of the venture to Rosneft.

Rosneft, which has made no secret of its desire to expand, is likely to conclude one or both deals, giving it effective control over TNK-BP. An announcement on Rosneft's plans could be made Thursday, the first day that BP is contractually allowed to strike a formal agreement to sell its share of TNK-BP.

Soon after Vladimir V. Putin returned for a third term as president this spring, BP opened talks with Rosneft to sell the British company's stake in TNK-BP.

The joint venture between BP and the Russian oligarchs has been highly profitable for both sides.

But the venture has been plagued by infighting, and BP has tried for years to form a partnership with a state company instead. In July, BP announced it had begun talks to sell its half of TNK-BP to Rosneft. No price was disclosed, but BP estimated the stake was worth $30 billion.

BP's partner, a consortium of post-Soviet billionaires known as AAR, has tried to prevent a breakup, lest it be left with a rump of a company, deprived of access to BP's technical skills and vulnerable to a state takeover on unfavorable terms. AAR refers to the three business groups in the consortium: Alfa Group, Access Industries and Renova Group.

After exploring buying BP's half of the venture, AAR decided to open talks to sell its own share of TNK-BP to Rosneft for about $28 billion, according to reports by Sky News and other outlets on Wednesday.

It is unclear whether Rosneft wants to buy all of TNK-BP or simply whichever half it can get at a better price. Either way, a deal would free BP from its troubled partnership with the oligarchs and allow it to strengthen its ties to Rosneft, which has close relations with the Putin government.

Arkady V. Dvorkovich, a Russian deputy prime minister, lent credence to the reports of a deal between Rosneft and AAR in comments Wednesday carried by the Russian Information Agency, but said the government had received no formal notice of Rosneft's plans.

"I know that commercial talks are ongoing, and as far as I have heard a letter of intent has been signed, while there is no legally binding document yet," Mr. Dvorkovich said. "There have not been any requests for approval so far, at least formal."

BP executives in London declined to comment on the reports, as did Rosneft and the Russian partners.

If either TNK-BP deal is finished, it would open a new chapter in the history of how the Kremlin and the post-Soviet clique of the ultrarich called the oligarchs do business together.

More often than not recently, such deals have led to greater consolidation of the oil sector under state control, a policy Mr. Putin favors that has partly reversed the privatizations encouraged by the West after the Soviet collapse.

Rosneft, formed from the detritus of the Soviet oil industry that was passed over for privatization in the 1990s, rose to become the country's largest oil company again after taking over the fields of the now imprisoned tycoon Mikhail B. Khodorkovsky.

The buyout of the AAR billionaires — Mikhail Fridman, Viktor F. Vekselberg and Len Blavatnik, and German B. Khan — would be the largest expansion of the state oil sector here since the oligarch Roman A. Abramovich sold his oil company, Sibneft, to Gazprom in 2005 for $13 billion.

The leaked announcement, like much else surrounding BP's business here, only underscored the wobbly foundation of even major business ventures in Russia. Under a contract with BP, no announcement should have been made for months.

The billionaires, under the shareholder contract, were not formally permitted to agree to a sale until a grace period of obligatory good-faith negotiations with BP expires.

Rosneft, in fact, may not have the money to buy out both partners in TNK-BP, said an investment banker with knowledge of the long-running talks about TNK-BP's future. BP had already been in talks about accepting Rosneft shares in lieu of cash as partial payment for its stake.

Morgan Stanley, in a research note issued on Wednesday, suggested that BP might benefit over the long term even if it could not sell its part of the venture, because the TNK-BP structure would probably keep paying high dividends even as it were absorbed into Rosneft.

TNK-BP now pays roughly 13 percent dividends on shares. The payments could help Rosneft finance its purchase.