Russian state-owned Vnesheconombank paid oil firm Rosneft $3.4 billion for 50 per cent of Yukos' former production unit, Tomskneft, a Rosneft document showed.

A bond prospectus obtained by Reuters, said Rosneft also paid $3.4bn to acquire other Yukos assets, including the firm's Moscow headquarters, its trading firm, Trading House Yukos-M, fuel stations and refined products depots. Rosneft did both transactions on June 25.

Rosneft bought Tomskneft, the 230,000 barrels per day East-Siberian unit, at a state-forced auction to sell the bankrupt Yukos' assets on May 3.

The purchase of Tomskneft and two smaller Yukos production units means Rosneft outstrips Lukoil as Russia's top oil producer and bolsters Rosneft's reserves.

Analysts said that the sale of a part of Tomskneft was a forced step for Rosneft, aimed at cutting its debt of $25bn, but was a profitable deal for the oil company.

"If 50pc in Tomskneft cost $3.4bn, it means that the whole unit costs $6.8bn. But Rosneft paid the same ($6.8bn) for the whole lot, which also included two refineries," said Artyom Konchin from Aton brokerage.

Rosneft paid 175.7bn roubles ($6.83bn) for the lot of Yukos' East Siberian assets, which along with Tomskneft included a smaller production unit, East Siberian Oil and Gas Company, and the Angarsk and Achinsk refineries.

Konchin suggested that Rosneft might have agreed with Vnesheconombank to hold the Tomskneft's shares until Rosneft is in better financial position to buy them back.

But he also did not rule out that the bank might decide to sell the stake on to a company such as gas monopoly Gazprom, whose oil unit, Gazprom Neft, develops deposits close to Tomskneft's.

Rosneft president Sergei Bogdanchikov said last week the firm planned to slash its debt to $15bn by 2010 and might sell some non-core assets bought in the Yukos auctions.

The firm plans the first issue of $2bn to $3bn under a $15bn Medium Term Note programme this month.