MOSCOW, March 14 (Reuters) - Russia's top natural gas producer Gazprom GAZP.MM said on Friday it has signed deals securing the supply of pipes for the second stage of the underwater South Stream pipeline. It said that the supply deals, worth a total of 800 million euros, were signed with Japan's Marubeni 9763.T , Itochu 8001.T , Sumitomo 8053.T Russia's OMK and Severstal CHMF.MM . The 2,400 km (1,500-mile) South Stream pipeline, which Gazprom says will be fully operational by 2018, would be able to supply over 60 billion cubic metres (bcm), or almost 15 percent of Europe's annual gas demand, pumping gas through the Black Sea into southeastern Europe. (Reporting by Vladimir Soldatkin; Editing by Steve Gutterman) ((vladimir.soldatkin@thomsonreuters.com)(+7 495 775 12 42)(Reuters Messaging: vladimir.soldatkin.thomsonreuters.com@reuters.net)) Keywords: RUSSIA GAZPROM/PIPELINE