Rosneft was on the verge on Thursday of becoming the world's biggest public oil trader by buying BP's half interest in the troubled but rich TNK-BP joint venture in Russia.
State-owned Rosneft waited until an 0800 GMT deadline before making a cash-and-shares offer which Dow Jones Newswires valued at $25 billion (19 billion euros) -- about three billion dollars less than indicated in earlier press reports.
That deal is set to be followed with a buyout of BP's four ex-Soviet billionaire partners for an even larger sum of up to $29 billion.
Completing the deal would bring the Russian state its greatest control of the oil sector since the Soviet era and re-define how the market operates under President Vladimir Putin.
The TNK-BP agreements culminate a year of tumult at Russia's third-largest oil company that highlighted the problems Western majors had in settling in for profitable business with Putin and his coterie of former spies in control.
The Financial Times reported that under Thursday's deal will see BP raise its stake in Rosneft from 1.4 percent to between 10 and 20 percent.
But Rosneft stressed that both deals were still only at the draft stage and did not imply necessary agreement.
"We continue to evaluate a number of opportunities and will update the market as and when appropriate," a Rosneft spokesperson said in an e-mailed statement to AFP.
And BP spokesman in London said only that "we are reviewing options including any offers that we have received."
Rosneft and the Soviet-born oligarchs who own the other half of TNK-BP had signed a memorandum of understanding on Tuesday which would result in Rosneft acquiring the Russian tycoons' stake at a later date.
The billionaires have experienced strained ties with the Russian government and were forced to concede that a company run by them and Rosneft jointly would most likely fail to function and require them eventually to sell their stake.
Various reports put the value of the deal with the tycoons -- to be completed most likely after the BP swap is set in place -- at $28-$29 billion.
The expected arrangement would make Rosneft the world's biggest publicly traded oil producer with daily crude output of more than three million barrels a day -- far more than current leader ExxonMobil's 2.3 million barrel figure.
It should also relieve tensions at one of Russia's most profitable oil companies -- a venture BP launched in 2003 to expand its presence in a country with the world's biggest oil and natural gas output.
TNK-BP has generated tens of billions of dollars and was responsible for more than a quarter of BP's crude output last year.
Yet tensions reached breaking point over BP's early 2011 efforts to strike a separate Arctic oil tie-up with Rosneft that the tycoons coveted for themselves.
That Arctic deal was blocked by the billionaires in a shock move that only deteriorated their relations with both Rosneft and BP still further.
BP chief Bob Dudley meanwhile has insisted that he still sees a future in Russia and has been willing to cede shares in TNK-BP in favour of a profitable long-term arrangement with Rosneft.
The deal should also see the state assert its greatest control of Russian oil sector since the Soviet era and work toward President Vladimir Putin's dream of national giants effectively running sectors of the economy for the Kremlin.
Finances still stand in the way of the arrangement because they would require Rosneft to go into heavy debt.
"It is unclear if Rosneft would be interested in acquiring the whole of TNK-BP, but doing so would likely result in much higher leverage and potentially additional negative rating action," Fitch Ratings said in an analysis of Rosneft's current investment-grade status.
A source close to TNK-BP oligarchs said their part of the arrangement could stretch out over an especially long period "because there is no financing yet."