ST PETERSBURG (Reuters) - Russian state-controlled oil company Rosneft has committed to supply China with 365 million tonnes of oil over 25 years in a deal worth around $270 billion, chief executive Igor Sechin said on Friday.

Sechin said the new supplies would start in July.

Russia already supplies China with 15 million tonnes (300,000 barrels per day) of oil annually via an Asian pipeline.

On Thursday, Russian President Vladimir Putin said Rosneft planned to sign a deal to supply China with oil worth $60 billion.

"This is one of the elements of the deal," Sechin, a long-standing Putin ally, told reporters.

Industry sources have told Reuters that Rosneft may secure up to $30 billion in prepayments from China as part of the deal.

Business daily Vedomosti said on Friday that the $60 billion which Putin referred to could be an advance payment secured by Rosneft against future oil supplies.

Sechin declined to comment on the report.

Rosneft and Russian oil pipeline monopoly Transneft secured a $25 billion loan from China in 2009 when Transneft was constructing the East Siberia - Pacific Ocean pipeline.

(Reporting by Denis Pinchuk; Writing by Vladimir Soldatkin, Editing by Alessandra Prentice and Mark Potter)

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