* Renaissance closes offer today
* Free float almost fully eliminated
* Gazprom needs to agree with major owners to gain control
MOSCOW, June 17 (Reuters) - Russian gas export monopoly Gazprom raised its stake in Sibir Energy to 33.5 percent, becoming the largest stakeholder in the mid-sized oil firm by almost fully buying out all minority shareholders.
Gazprom is widely believed to be aiming for control in Sibir, which ran into troubles last year after its main shareholder, Shalva Chigirinsky, was forced to put up his stake as collateral with state bank Sberbank to meet obligations on Western loans.
Many of Russia's once mighty oligarchs found themselves in a similar situation last year after borrowing heavily to fuel growth.
The Kremlin threw lifelines to many of them and helped repay foreign loans but took large assets and collateral to guarantee repayment.
Many analysts believe the Kremlin will tighten its grip over the economy by becoming the ultimate owner of many assets if some oligarchs fail to pay back their debts.
On Wednesday, Renaissance Capital brokerage said in a statement Gazprom's oil arm, Gazprom Neft, had raised its stake in Sibir to 33.5 percent from 28.5 percent.
Going above 30 percent automatically triggers a mandatory buyout under Russian law, but Renaissance said it would close the offer on behalf of Gazprom Neft at 1300 London time on Wednesday in a sign it had already bought out almost all minority shareholders.
Chigirinsky and his partner Igor Kesayev jointly control around 47 percent, which are now collateralised with Sberbank, while the government of Moscow owns 18 percent, which reduces the potential outstanding free-float to just 1.5 percent.
Market sources have said Gazprom wants to buy out the stakes of both Kesayev and Chigirinsky.
The fate of the latter is complicated by the fact that another Russian businessman, Ruslan Baisarov, claims ownership of it while Chigirinsky has won a British court order prohibiting Gazprom and Gazprom Neft from buying those shares until their ownership becomes clear. (Editing by Jon Loades-Carter)