Top Russian oil firm Rosneft said it would sell about $6 billion worth of refined products to BP, establishing the British major among its biggest buyers after years without any such deal.

The latest transaction comes on top of a $5.3 billion deal for BP to buy crude, agreed earlier this year. Both came only months after Rosneft hired a former senior BP trader, Marcus Cooper, as head of its own trading arm in Geneva.

BP has become Rosneft's second-largest shareholder, behind only the Kremlin, after Rosneft took over BP's Russian venture TNK-BP for $55 billion in March.

But the participation in Rosneft's ownership was slow to translate into cooperation in upstream projects, while Exxon Mobil signed a flurry of deals with Rosneft to tap the Russian Arctic together.

BP has also been absent for many years from the list of major buyers of Rosneft's oil and refined products, while rivals such as Shell have been among the biggest players in taking oil exports from the world's largest producer.

Over the past year, Rosneft also has sold large amounts of crude to trading houses Glencore, Vitol and Trafigura as they have lined up $11.5 billion worth of pre-payment, which helped Rosneft ease the burden on its balance sheet.

MARCUS COOPER

BP had its first breakthrough in October, when it signed deals to buy $5.3 billion of crude oil from Rosneft.

On Tuesday, Rosneft said in a regulatory filing it also agreed to sell up to 3.2 million metric tons of fuel oil worth up to $2.6 billion to BP Singapore from the Far East ports of Nakhodka or Vanino between November 2013 and December 2014.

It will also sell up to 1.44 million metric tons of diesel to BP worth up to $1.77 billion from the Black Sea port of Tuapse; up to 2 million metric tons of fuel oil to BP worth as much as $1.62 billion from the Baltic port of Ust-Luga; and up to 60,000 metric tons of naphtha worth $65 million from Tuapse.

Rosneft and BP's Moscow office declined to comment on the deals.

Cooper took over trading operations at Rosneft's Geneva office after years at BP, including a senior position at its office in Singapore.

A Singapore-based fuel oil trader with a Western trading house said he expected most of the volumes from the Rosneft contract to head to China.

"BP will have a bigger trading presence in the straight-run fuel oil market in China," he said.

BP is buying from Rosneft straight-run fuel oil, which is typically processed by smaller, independent refineries in China.

"BP is taking a view on the straight-run market next year," another Singapore-based fuel oil trader said, as refineries are lowering crude distillation runs because of low refining margins and are maximizing runs at secondary units.