British oil company BP said on Monday it had yet to reach an agreement on the sale of its 50% stake in Russian venture TNK-BP to state-run Rosneft.

"BP confirms it is in advanced discussions with Rosneft regarding the sale of its 50% interest in TNK-BP. No agreement has yet been reached. A further announcement will be made if and when an agreement is reached," it said in a statement.

According to sources familiar with the situation, Rosneft and BP are preparing to announce a deal worth over $25 billion that could give the British oil company a stake of between 16 and 20% in the state-controlled Russian energy firm, sources familiar with the situation said.

The agreement, which has yet to be finalised but which could be made public on Monday or Tuesday, folds BP's half of TNK-BP , Russia's third-largest oil company, into Rosneft, in exchange for cash and Rosneft stock.