By Promit Mukherjee

MUMBAI, Aug 21 (Reuters) - Russian oil major Rosneft, oil trader Trafigura and Russian fund UCP closed their $12.9 billion purchase of Indian refiner Essar Oil on Monday, giving the companies a foothold in a country that is among the world's fastest growing oil users.

The purchase is the biggest foreign acquisition ever in India and Russia's largest outbound deal. It helps Russia deepen economic ties with India that stretch back to the Soviet era.

Rosneft, Trafigura and UCP purchased a 98.26 percent stake in Essar Oil in a deal first announced in October. The rest of Essar will be held by retail investors. urn:newsml:reuters.com:*:nL4N1CJ484

"(Rosneft) has entered the high-potential and fast-growing Asia Pacific market," Rosneft's Chief Executive Officer Igor Sechin said in a statement. The deal will enable Rosneft to improve the efficiency of fuel supplies to other nations in Asia, he added.

India's oil demand is expected to rise by an average 5.9 percent through 2020, among the fastest in the world, according to a report last month by Goldman Sachs.

To capitalize on that growth, Rosneft and its partners acquire Essar's oil refinery in Vadinar that can process 400,000 barrels a day of crude. The refinery is located in the western Indian state of Gujarat and the deal includes a port, a power plants and 3,500 fuel stations.

Tony Fountain, chairman of the new Essar Oil entity, said his firm is taking over $5 billion in debt and will also settle over 2 billion euros owed to Iran for past oil purchases.

He said the company will expand its fuel station fleet to 6,000. Rosneft also plans to double the Vadinar refining capacity and build a petrochemical facilities.

For Essar Group, the deal helps it cut its debt by about $11 billion, Prashant Ruia, director, Essar Capital, told a news conference.

Trafigura and Rosneft are the latest international companies after Royal Dutch Shell RDSa.L and BP BP.L to enter the Indian fuel retailing market.

The Russian major's stake in Venezuelan upstream assets and oil purchase contracts with state-owned PDVSA will help in improving the economics of the Indian refinery, Rosneft said on Monday.

Reuters last year reported that Rosneft planned to supply Venezuelan oil to the Vadinar refinery.

(Writing by Nidhi Verma; Editing by Michael Perry and Christian Schmollinger) ((nidhi.verma@thomsonreuters.com; +91 11 49548031; Reuters Messaging: nidhi.verma.thomsonreuters.com@reuters.net))