MOSCOW (Reuters) - Russia's largest oil producer Rosneft said on Sunday it had found its first oil field in the Laptev Sea in the eastern Arctic, making a breakthrough in the search for hydrocarbons in the harsh and far-flung region despite Western sanctions.

Rosneft and its partners plan to invest 480 billion roubles ($8.4 billion) in developing Russia's offshore energy industry in the next five years, part of a drive to boost output from new areas.

The company has sought tie-ups with several global oil players to develop Russia's offshore regions. But a deal to work in the Kara Sea in the western Arctic with U.S. company Exxon Mobil was suspended in 2014 after the imposition of Western sanctions against Moscow.

"The result of the drilling at the Khatanga licence block allows Rosneft to be considered the discoverer of [oil] fields in offshore Eastern Arctic," the company said in a statement.

Most Russian oil output comes from western Siberia, where fields are depleting, pushing producers to look for new regions. Sanctions complicate the process, barring Western companies from helping with Arctic offshore, deepwater and shale oil projects.

The Arctic offshore area is expected to account for between 20% and 30% of Russian production, one of the world's largest, by 2050.

Rosneft owns 28 blocks in the Arctic offshore area with combined estimated resources of 34 billion tonnes of oil equivalent.

There is only one offshore platform in the Russian Arctic, Prirazlomnoye, operated by Gazprom Neft, which plans to produce 2.6 million tonnes (52,000 bpd) this year.

Analysts say oil production in the region - apart from Prirazlomnoye - is years away and may start only in the mid-2020s.

Rosneft has been working in the Laptev Sea since 2014. It values the hydrocarbon resources of the sea at around 9.5 billion tonnes of oil equivalent.