BP's board on Friday will consider a bid by Rosneft to buy the British company's stake in TNK-BP, news reports said.
The cash and stock offer is worth more than $25 billion.
The board may have to reconvene for a more in-depth consideration of some aspects of the offer, a source said, Reuters reported.
Rosneft has also reportedly made a $28 billion offer for the other half of TNK-BP, a stake held by four billionaires.
Both of the deals are subject to government approval at a time when Prime Minister Dmitry Medvedev is pushing for the state to cede its assets in privatization deals.
A takeover of TNK-BP in its entirety would, thus, require Rosneft to cough up more than $50 billion worth of cash and equity.
In that event, Rosneft would expand to control about half of the Russian oil industry. Rosneft is already the country's leading crude producer, while TNK-BP is the third-largest.
Gazprom Neft, another state-controlled oil company, is the industry's fifth-largest in Russia. It used to be a private company called Sibneft before Gazprom agreed to buy the asset.
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