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Russian energy giant Rosneft says it has discovered oil with its US project partner Exxon Mobil at a controversial well in the Arctic.

Drilling was completed in record time, it said, but questions remain about how quickly the well can be developed.

Exxon has said it will "wind down" the project following US sanctions against Russia over its actions in Ukraine.

Environmentalists have campaigned hard against drilling for oil in the pristine region.

"Rosneft successfully completed the drilling of the northernmost well in the world - the Universitetskaya-1 well in the Arctic," the company said in a statement.

Rosneft boss Igor Sechin, himself a target of US sanctions, said the well had produced "an astonishing sample of light oil".

He estimated the well could access 100 million tonnes of oil and 338 billion cubic metres of gas.

"This is an outstanding result of the first exploratory drilling on a completely new offshore field," he continued.

"This is our united victory - it was achieved thanks to our friends and partners from Exxon Mobil, Nord Atlantic Drilling, Schlumberger, Halliburton, Weatherford, Baker, Trendsetter, FMC."

Experts said more testing would need to be carried out before an accurate picture of recoverable reserves in the region could be established.

They said it was unclear how the project would develop given US sanctions that prohibit US companies from participating in joint ventures with Russian energy companies.