The London-based oil company had set a deadline of 9:00 am Thursday morning for bids for its stake in TNK-BP. It had earlier said it was in talks with Rosneft about a possible deal, while the only other likely bidder was Alfa-Access-Renova, BP's partner in TNK-BP.

A source close to AAR said Thursday that the investment vehicle, owned by four Russian-born billionaires, had not to bid for BP's stake. AAR on Wednesday agreed the terms of its own $28 billion deal to sell its 50% share of TNK-BP to Rosneft.

"BP is reviewing options, including any offers that we have received," a BP spokeswoman said. BP declined to give any further details. Rosneft is expected to offer between $25 billion and $28 billion in cash and stock for BP's 50% TNK-BP holding.

Rosneft had planned to offer BP $10 billion to $15 billion in cash and a stake in Rosneft of about 12.5%, according to reports from Russia, though sources said that the bid could increase in the wake of the $28 billion offer for AAR's stake.

Rosneft CEO Igor Sechin flew into London on Wednesday to negotiate the terms of a deal with BP representatives and to meet with bankers to discuss debt for the offer. BP's board will meet Friday to consider the bid and BP is unlikely to make any announcement on the terms of the deal until after that board meeting.

Acquiring TNK-BP will propel Rosneft into the top tier of global oil producers, pushing its daily output up from about 2.3 billion barrels to just over 4 billion. That would make it the world's largest publicly traded oil producer by crude oil output. Exxon-Mobil Corp., the world's most valuable oil company, produced about 2.3 billion barrels of oil in 2011, though its total "oil-equivalent production", which includes natural gas production, amounted to about 4.5 billion oil-equivalent barrels per day.

Rosneft is 75% owned by the Russian state, while about 15% of its shares are in free float, according to the company's website.

TNK-BP, which operates upstream assets in Sibera and the Volga Urals areas of Russia, made net profit of about $9 billion in 2011 and has about 13.8 billion barrels of proven reserves. TNK-BP, whose ownership is split equally between BP and AAR, owns 95% of the listed TNK-BP Holding.

The cash portion of Rosneft's bid for TNK-BP will stretch the finances of the Russian oil giant, but the Russian state could help it overcome that constraint.

"For Rosneft to buy both AAR and BP to form a fully state-owned oil champion would be the cleaner solution for the Russian state, and is likely to require further state injection into Rosneft, given that the company had previously been sounding out the market for a loan to buy part of BP's share," RBC Capital Markets Corp.'s London-based oil analyst Peter Hutton noted.

BP already owns 1.5% of Rosneft. The London-headquartered oil company last year failed to secure a share swap that would have increased its stake in the Russian company and underpinned a new joint venture to prospect for oil in the Russian arctic.

That plan was blocked by AAR, which won a legal victory that forced BP to conduct all of its Russian operations through TNK-BP. Rosneft and BP subsequently offered to buy AAR out of TNK-BP for $32 billion, an offer that was rejected by the investment consortium.

BP in June said that it had put its stake in TNK-BP on the block after receiving offers for the asset, which by that stage it considered unmanageable because of the complete breakdown in relations between itself and AAR.

AAR had initially indicated that it would seek to buy BP's holding in TNK-BP, but abandoned the idea after striking its own deal with Rosneft on Tuesday. Sources have warned that the Rosneft/AAR deal remains tentative and could still collapse, depending on the final terms and on Rosneft's ability to fund the all-cash bid.

Shares in BP traded Thursday at 451 pence ($7.28), up 2.65 pence on their previous close. Shares in Rosneft traded at $6.94, up less than 1% on their previous close.