A company that Rosneft recently sold to a U.S. entrepreneur bought Yukos' international unit at a forced auction Wednesday, as former Yukos managers vowed to pursue a legal challenge to salvage part of what was once the country's largest oil firm.

Promneftstroi, recently acquired by businessman Steven Lynch, won the unit, Yukos Finance, for 7.8 billion rubles ($305 million) after five minutes of bidding.

The winning bid fell well below the $2.7 billion value that former Yukos managers place on the unit, and was just 240 million rubles ($9 million) above the starting price.

Rosneft has acquired nearly all the main assets that once belonged to Yukos and as a result is now the country's largest oil producer. Yukos was declared bankrupt last year, felled by over $30 billion in back tax claims and the jailing of its former CEO, Mikhail Khodorkovsky.

The former London-based managers of Yukos called the auction "an elaborate farce."

"The 'winner' has won absolutely nothing," Claire Davidson, a spokeswoman for the former managers, said in a statement. "An illegal sale as a part of a sham bankruptcy is not a valid transaction."

Yukos Finance, registered in the Netherlands, holds Yukos' foreign assets -- a 49 percent stake in Slovak pipeline monopoly Transpetrol, revenue from the sale of the Mazeikiu Nafta refinery in Lithuania, and Intelligent Energy, a London-based fuel cell technology firm.

Rosneft said it had nothing to do with Wednesday's auction, which pitted Promneftstroi against Versar, an unknown firm that had participated unsuccessfully in two previous auctions.

"We did not take part in today's auction. We no longer have any relationship with the company," Rosneft spokesman Nikolai Manvelov said, referring to Promneftstroi. Rosneft "used to be affiliated" with the firm, he said, declining to provide details into how or when the relationship was severed.

Lynch, a U.S. citizen who founded and heads Monte Valle, a Moscow real estate firm, said Wednesday that he had recently bought Promneftstroi with a group of investors.

"I saw an opportunity here. We told the owners 'you don't want to be in this' ... and said we'd happily take it," Lynch said in a telephone interview.

Yukos Finance could still end up in Rosneft's hands, as Lynch said he might sell it, adding, "We have multiple plans for multiple scenarios."

Monte Valle has sold most of the assets it won in one of the first in a series of auctions that have taken place over the past five months as the state sells off the remains of Yukos.

On April 17, the firm won a lot comprising 10 electricity assets in western Russia, beating out Rosneft, Versar and another unknown firm with a 35.6 billion ruble ($138 million) bid.

Analysts speculated at the time that Monte Valle could be fronting for state-run gas monopoly Gazprom, which did not participate openly in any of the auctions amid legal concerns. Lynch declined to name the buyers of the power assets.

Nikolai Lashkevich, a spokesman for Yukos' bankruptcy receiver, declined to comment on who stood behind Promneftstroi's bid. "It's a case of 'the less you know the better you sleep,'" he said.

Yukos' former managers have mounted a series of legal challenges in Amsterdam and at the European Court of Human Rights in Strasbourg to declare the Yukos bankruptcy illegal.

"Of course we have plans for that," Lynch said, when asked whether Promneftstroi's new owners feared a legal challenge. "We evaluated the risk and made judgments," he said.

Lawyers for Yukos' former managers have asked a Dutch court to refuse to recognize the Yukos bankruptcy, which would invalidate the sale of Yukos Finance.

Barring a bilateral treaty, the Netherlands does not recognize foreign bankruptcies. Russia has no such treaty with the Netherlands.

Three lawyers representing Yukos' court-appointed bankruptcy receiver, Eduard Rebgun, attended Wednesday's auction.

"We recommended the participants conduct independent research to assess the legal risks," said Julia Romanova, a lawyer with the Moscow office of U.S. law firm Chadbourne & Parke. "The bidders know everything."

The other two lawyers, from Dutch law firm Simmons & Simmons, represent Rebgun in Dutch cases. An Amsterdam court is due to rule on Oct. 31 whether Yukos' former chief financial officer Bruce Misamore and former legal counsel David Godfrey were properly dismissed. That ruling would also affect whether the Dutch courts recognized Rebgun's leadership, Yukos spokeswoman Claire Davidson said.

"Whether anyone likes it or not, there is now a new owner," said Tim Osborne, the head of Yukos majority shareholder GML, formerly known as Group Menatep.

Khodorkovsky and Platon Lebedev, the former head of Menatep, are serving eight-year sentences on charges of fraud and tax evasion. They are currently awaiting a new trial on charges of multibillion-dollar money laundering and embezzlement that could keep them in prison for a further 25 years.

Khodorkovsky has accused Igor Sechin, the chairman of Rosneft's board of directors and a close adviser to President Vladimir Putin, of orchestrating the legal onslaught against him and Yukos.