Gazprom Neft, the country's fifth-largest oil producer, launched a new offer to raise its stake in Sibir Energy to as much as 30 percent, a bank representing the company in the buyout said Friday.

Meanwhile, rumors swirled that the state-owned corporation would purchase an additional stake from a major shareholder, effectively taking over the company.

Renaissance Capital, which is acting as Gazprom Neft's agent in the transaction, said the company would increase its position to no more than 30 percent of the British company's shares. Under British law, a stake of 30 percent would force Gazprom Neft to initiate a formal takeover of the company.

Gazprom Neft already owns a 16.95 percent stake in Sibir after snapping up 16 percent in April. Sibir's London-listed shares stopped trading in February after it became known that Shalva Chigirinsky, one of the firm's major private shareholders, owed the company $325 million.

A 30 percent stake would make Gazprom Neft the single largest shareholder in the company. Renaissance Capital said on Friday that it would begin buying the shares for Gazprom Neft immediately, without saying at what price the shares would be purchased.

Sibir later said in a statement that the previous purchase by Gazprom Neft, which took place last month, was conducted at 500 pence per share and encouraged shareholders to seek that price.

While Gazprom Neft has maintained that it only intends to be a minority shareholder in Sibir, Interfax reported Friday that the state-owned company planned to purchase a 23.3 percent stake in Sibir owned by businessman Igor Kesayev that has been held as collateral by Sberbank.

In addition to a stake in the Moscow Refinery, Sibir owns half of Salym Petroleum Development, a joint venture with Royal Dutch Shell that has been ramping up production at its Siberian field.

The Moscow government owns an 18 percent stake in Sibir Energy, while about 18 percent of the company's stock is in free float.

n Chigirinsky has handed over his French Riviera villa and a London mansion to Sibir, which is suing him for $400 million, Reuters reported Friday.

Sibir said Friday that it had "received a charge" over shares in the company, which controls Villa Marina Irina on France's Cap Martin, and a pledge of future profits from the sale of Hugh House in London's upscale Belgravia district.

Hugh House was listed for sale on an exclusive British property web site at ?45 million ($71.08 million).

Sotheby's could not comment immediately on Hugh House.

Sibir suggested that the property and shares handed over to the company would not cover the full amount owed, saying it was seeking further security from Chigirinsky.