Russia's top oil producer Rosneft signed a memorandum on Friday giving the world's largest crude importer China its first direct access to energy-rich East Siberian fields.

The preliminary agreement with China National Petroleum Corp (CNPC) aims to help Rosneft meet the terms of a $270-billion deal signed in June that doubles Russia's oil exports to its giant neighbour over 25 years.

It also underscores the accelerating shift of Russia -- the world's biggest combined producer of oil and natural gas -- from its traditional markets in Europe to regions in Asia with exploding consumption and energy needs.

"It is logical for Rosneft to be expanding in the direction where there is growth," said VTB Capital energy analyst Alexander Kirevnin.

Analysts believe that state-owned Rosneft -- the world's largest listed oil firm by output -- needs to make huge investments in its untapped East Siberian deposits to meet outsized Chinese demand.

The agreement announced by Rosneft on Friday will give the Russian firm a 51-percent stake and CNPC a 49-percent holding in a joint venture for upstream developments.

The venture will begin by expanding production at the Srednebotuobinsk field that Rosneft acquired fully on Monday and which sits close to the ESPO pipeline running across Eastern Siberia to China.

"The memorandum is another step in developing the strategic partnership between Rosneft and CNPC in various areas of cooperation," Rosneft chief executive Igor Sechin said in a statement.

"Our balanced strategic position will enable us to jointly develop and produce hydrocarbons, execute long-term supplies, jointly construct refining capacity and manage various assets," said Sechin.

Russia had previously resisted giving China direct stakes in its oil fields and instead preferred to strike long-term contracts in exchange for loans.

The deal signed in June in the presence of Russian President Vladimir Putin and Chinese Vice Premier Zhang Gaoli saw Rosneft receive a $70-billion upfront payment from CNPC for its promise to raise exports to by 300,000 barrels per day.

Rosneft said the Srednebotuobinsk field holds about 134 million tonnes of oil and gas condensate. The deposit is also believed to contain more than 155 billion cubic metres of gas.

Yet China's demand for energy stretches far beyond the resources now available in East Siberia -- a region left untapped by the Soviet Union and requiring massive investments by Rosneft at a time when it can ill afford to assume further debt.

Rosneft this spring closed a $55-billion takeover of its private Anglo-Russian rival TNK-BP and has also made acquisitions aimed at expanding its natural gas portfolio.

The big moves promoted Fitch Ratings on Friday to downgrade Rosneft's long-term foreign and local currency debt ratings to 'BBB-' from 'BBB'.

Fitch said that state support made Rosneft's outlook more stable than that of rivals with similar debt obligations.

But it still expressed worry that lags in the global economy may make it difficult for the firm to repay $38 billion in obligations maturing by 2015.

The Russian firm's hesitancy to assume still more risk means that CNPC will be responsible for financing most of the East Siberian development work.

But China's largest oil company has been eagerly seeking new investments in Russia -- seen as a stable source of energy against the backdrop of volatility in Africa and the Middle East.

CNPC in June also struck a deal to acquire a 20-percent stake in a liquified natural gas project in the Russian Arctic known as the Yamal LNG.

"China is Russia's most important economic partner," said Valery Nesterov of Sberbank CIB.

"Russia needs to expand its exports to China. And Russia needs Chinese investments in order to finally start developing our stagnating East Siberia and Far East."