(Adds Sechin's comment on Seadrill, detail)
* Rosneft may increase stake in Seadrill to 50 pct -Sechin
* Rosneft, Seadrill spin-off to team up in Russia
* Deal may value North Atlantic Drilling at $2 bln -Interfax
ST PETERSBURG, Russia/OSLO, May 24 (Reuters) - Russia's Rosneft could buy a controlling stake in the world's largest offshore driller, Norway's Seadrill, after saying it was in talks to acquire a significant stake in Seadrill subsidiary North Atlantic Drilling.
The announcement comes as Rosneft and North Atlantic Drilling signed a cooperation agreement on Saturday, one of several deals announced by Russia's largest oil firm during the St. Petersburg International Economic Forum, showing the keenness of multinationals to do business with Russia despite tensions over Ukraine.
The agreement is also a coup for Norwegian-born tycoon John Fredriksen, one of the world's richest men, who controls Seadrill and North Atlantic Drilling, as it gives him access to the Russian exploration market on a large scale.
"The agreement contemplates Rosneft acquiring a significant stake in North Atlantic Drilling Ltd. with the latter entering Russia's onshore drilling market, and signing of long-term contracts for onshore and offshore drilling," Rosneft said in a statement on Saturday.
Separately, Rosneft boss Igor Sechin told the Rossiya 24 TV channel his firm could take a stake of up to 50 percent in Seadrill over time, suggesting Fredriksen may further expand his reach in Russia.
"We're becoming Seadrill shareholders. At this stage we're discussing combining our service company RN Bureniye with Seadrill and handing it some of our orders. In time we could increase our stake to 50 percent," Sechin was quoted as saying by the Interfax news agency.
Seadrill is the world's biggest offshore oil driller by market capitalisation, ahead of Transocean. Fredriksen, nicknamed "Big Wolf" for his business instincts, is Seadrill's largest shareholder with a stake of 24.54 percent.
Seadrill is the crown jewel in Fredriksen's business empire, that ranges from shipping to fish farming.
NORTH ATLANTIC DRILLING DEAL
North Atlantic Drilling specialises in drilling in harsh environments like the Arctic. Seadrill spun it off and listed it on the New York Stock Exchange in January. At the time, the initial public offering raised $125 million.
The partnership agreement involves developing Rosneft's offshore and onshore projects until at least 2022.
Rosneft provided no further details, but Interfax reported the deal could value North Atlantic Drilling's capitalisation at around $2 billion.
"We have sought to access the growth opportunity represented by the Russian market for several years, and we are very pleased to have reached an agreement with Rosneft for this landmark transaction," North Atlantic Drilling CEO Alf Ragnar Lovdal said in the statement. (Reporting by Alexei Anishchuk in St Petersburg and Gwladys Fouche in Oslo; Writing by Lidia Kelly in St Petersburg and Gwladys Fouche in Oslo, Editing by Timothy Heritage and David Evans)