UPDATE 2-Greece joins Russia's South Stream gas pipeline

(Adds routing, Greek amounts)

MOSCOW, April 29 (Reuters) - Greece formally signed on Tuesday a deal to join Russian gas export monopoly Gazprom's (GAZP.MM) 10 billion euros ($15.58 billion) South Stream gas pipeline.

The pipeline, which will be jointly built by Gazprom and Italy's ENI (ENI.MI), will take 31 billion cubic metres (bcm) of Russian gas to southern Europe per year after it is commissioned in 2013.

"The signing of the agreement to construct the Greek part of the South Stream pipeline has become the most important result (of the talks)," Russian President Vladimir Putin said after meeting Greek Prime Minister Costas Karamanlis at the Kremlin.

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

Analysts see the South Stream project as posing a major challenge to the rival U.S. and EU-backed Nabucco pipeline scheme. Tuesday's deal with Greece struck yet another blow.

Under Nabucco, gas would come from ex-Soviet Azerbaijan to south Europe via Turkey in an EU effort to diversify energy sources away from reliance on Russia, though experts say the pipeline's fruition is becoming increasingly more difficult as Azeri reserves are questioned.

"Such (a) truly forward-looking and pragmatic approach by the Greek leadership will bear solid dividends for Greece, the Balkans and Europe as a whole very soon," Putin said after Russian Energy Minister Viktor Khristenko and Greek Minister of Development Christos Folias signed the agreement.

Under the South Stream scheme, Russian gas will go from the Novorossiisk port, travel 900 kms under the Black Sea, re-emerge on the Bulgarian coast and then continue through one of two onshore routes.