This gives Rosneft more than three-quarters of the shares in Yukos' Yuganskneftegaz production unit, which produces about 11% of Russia's oil, reports said.

BaikalFinansGroup bought those shares at a government auction on Sunday.

"Rosneft bought 100% of shares previously owned by BaikalFinansGroup," an unnamed Rosneft official was quoted as saying by the Interfax agency.

In the last few months, the Russian government has announced it would fold Rosneft into the oil assets of the natural gas giant, Gazprom, and form a new company.

That company, called Gazpromneft, had been widely expected to purchase the Yuganskneftegaz production unit at Sunday's auction, but was stopped just days earlier by a US bankruptcy court order.