By James Marson 

MOSCOW--Russia's state oil giant OAO Rosneft sold a minority stake in an eastern Siberian oil-and-gas field to BP PLC for $750 million and bought a stake in a German refinery from France's Total SA, underscoring the limitation of Western sanctions that have targeted Rosneft and its chief executive, a key ally of President Vladimir Putin.

Rosneft Chief Executive Igor Sechin, Russia's most-powerful oil official, signed a raft of deals Friday at the St. Petersburg International Economic Forum, including further production, exploration and refining agreements with BP.

U.S. and European sanctions over Russia's interventions in Ukraine prevent Rosneft from raising credit in the West or accessing Western technology for tapping shale and arctic reserves, but don't prevent asset sales and purchases. The U.S. has also imposed personal restrictions on Mr. Sechin.

The deals announced on Friday cement the strategic alliance between Rosneft and BP, which owns a nearly 20% stake in the Russian company. BP is taking a 20% stake in Taas-Yuryakh Neftegazodobycha in Eastern Siberia, creating a new joint venture, the companies said. Russia has been reluctant to agree to foreign companies' taking stakes in oil and gas fields, but the deal allows Rosneft to raise much-needed capital.

"It's a form of indirect financing," said Ildar Davletshin, an energy analyst at Renaissance Capital in Moscow. "Russia has been reluctant to do this until now, but the time is different."

Rosneft has also been trying to sell a stake its large Vankor field to foreign companies, including China, but with little success.

For BP, the deal is a chance to add reserves at a cheaper price than in other areas of the world, Mr. Davletshin said. Rosneft has said that crude produced at the Taas-Yuryakh unit will be sent to refineries in Russia's far east and in China.

"BP will continue to seek attractive investment opportunities to develop Russia's substantial resources, whilst continuing to comply with international sanctions," BP Russia President David Campbell said.

The companies will also explore an associated area of mutual interest in the region, covering 115,000 square kilometers. In addition Rosneft and BP have agreed to explore two additional areas in the West Siberian and Yenisey-Khatanga basins, covering a combined area of about 260,000 square kilometers, committing them to establish new joint ventures to obtain licenses and perform exploration activities. Any joint ventures will be owned 51% by Rosneft and 49% by BP.

The companies plan to reorganize the German Ruhr Oel GmbH refining joint venture by dividing shares in four refineries and associated infrastructure.

As a result, Rosneft will double its shareholding in the Bayernoil refinery to 25% from 12.5%, MiRO refinery to 24% from 12%, and the PCK Raffinerie to 37.5%, from 18.75%.

BP in exchange will consolidate 100% of the equity of the Gelsenkirchen refinery and the solvent production facility DHC Solvent Chemie.

Mr. Sechin also signed a deal Friday to buy a 16.67% stake in the PCK Raffinerie GmbH in Germany from Total. Total said the sale, which it put at $300 million, was part of its plan to divest refining activities and focus on oil and gas extraction. Rosneft already held an indirect stake of 18.75% in the refinery.

Ian Walker in London and Inti Landauro in Paris contributed to this article.

Write to James Marson at james.marson@wsj.com

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