UPDATE 4-Gazprom Neft acquires Sibir stake, trumps TNK-BP

* Expects to own 16 pct-plus of Sibir after 500p offer

* TNK-BP cancels 430 pence a share book-build offer

* Sibir says third party approach now withdrawn (Adds comments, details, background)

By Julie Crust and Robin Paxton

LONDON/MOSCOW, April 23 (Reuters) - Gazprom Neft (SIBN.MM: Quote, Profile, Research), Russia's No. 5 oil producer, is set to acquire over 16 percent of Sibir Energy (SBE.L: Quote, Profile, Research) after trumping rival TNK-BP with a bid valuing the London-listed firm at about $2.8 billion.

The oil arm of Russian state-controlled gas giant Gazprom (GAZP.MM: Quote, Profile, Research) offered 500 pence each to buy shares in Sibir, which has become a takeover target since announcing two months ago it was pursuing its main shareholder for a $325 million debt.

"We are pleased to have become a significant minority shareholder in a company that we regard as a world-class asset," Gazprom Neft Chief Executive Alexander Dyukov said.

The offer, which prompted TNK-BP to drop its 430 pence book-build offer and an unnamed third party to withdraw, was almost triple the price at which shares in Sibir were suspended in February.

It was organised by Russian investment bank Renaissance Capital, which announced the deal in a statement on Thursday.