"Russia's weak domestic economy needs help from China. The crude trade between the two countries has become an effective tool for their strategic and political cooperation," he said.

It is the first time since October 2005 that Russia has become China's top crude supplier, but Moscow has been forced into finding new markets for its crude amid Western sanctions over its ongoing conflict with Ukraine.

Energy cooperation between the two countries is not limited to crude.

In November, they signed a 30-year gas supply contract, making China the biggest consumer of Russian gas.

The new supply will come through China's western border and is in addition to a $400 billion deal between the nations to deliver 38 billion cubic meters of gas annually to China via an eastern route, starting in 2018.