BELGRADE, December 5 (RIA Novosti) - Gazprom's oil arm Gazprom Neft will buy a 51% stake in Serbia's NIS for $400 million by the end of the year, the Russian energy giant's CEO said on Friday after talks with the Serbian president.

"An agreement was reached today to sign three deals by the end of the year," Alexei Miller said, referring to Gazprom Neft's acquisition of state-owned Naftna Industrija Srbije, Gazprom's $500 million investment in a gas storage facility in Serbia, and the construction of the Serbian segment of the South Stream pipeline.

Under the South Stream project, a 400-km (248 mile) leg will be built in Serbia for Russian natural gas supplies to and via the Balkans.

Miller said some technicalities remained to be settled for the signing of the three deals, initialed in Moscow in January.

"[Serbian President Boris] Tadic confirmed the documents will be signed by the end of the year," he said.

NIS produces around 1 million metric tons (7.3 million barrels) of crude annually, refines 7 million metric tons (51 million barrels) and has Serbia's largest network of filling stations.

Gazprom Neft, known as Sibneft before it was taken over by Gazprom in September 2005, produced 32.7 million metric tons (240 million barrels) of crude and posted a US GAAP net income of $4.14 billion in 2007