CORRECTED - UPDATE 1-Hungary joins Russia's South Stream gas pipeline

(Corrects speaker in paragraph 11 to Gyurcsany)

(Adds more quotes, background, details)

MOSCOW, Feb 28 (Reuters) - Hungary formally signed on Thursday a deal to join Russian gas export monopoly Gazprom's (GAZP.MM: Quote, Profile, Research) 10 billion euros ($15.10 billion) South Stream gas pipeline. The pipeline, which will be jointly built by Gazprom and Italy's ENI (ENI.MI: Quote, Profile, Research), will take 30 billion cubic metres (bcm) of Russian gas to southern Europe a year.

"Stable cooperation in the oil and gas sectors is a joint contribution by Russia and Hungary into Europe's energy security," Russian President Vladimir Putin said after the signing ceremony in the Kremlin.

The deal was first agreed earlier this week in Budapest by Hungarian Prime Minister Ferenc Gyurcsany and Russian First Deputy Prime Minister and chairman of Gazprom Dmitry Medvedev.

Putin's protege and Russia's likely next leader, Medvedev is widely expected to win in this Sunday's presidential election.

"I'm convinced that these agreements serve the interests of Hungary, the EU and Russia... I found First Deputy Prime Minister Medvedev a tough but constructive negotiating partner," Gyurcsany said in the Kremlin.

Last month Russia agreed to route the South Stream through Bulgaria and Serbia, marking major victories in a 'pipeline war' with the European Union.

Analysts see the South Stream project as posing a challenge to the rival U.S. and EU-backed Nabucco pipeline scheme.