Russia became China's top crude oil supplier last month for the first time in a decade, Bloomberg News reported, as Moscow seeks new markets for its crude following western sanctions.

China imported a record 3.92 million tonnes of crude from its northern neighbour in May, it said, citing official customs data released on Tuesday.

Imports of Russian oil increased 20 percent in May from the previous month to reach the equivalent of 927,000 barrels per day, the report said.

China's oil imports from Angola increased in May to reach 3.26 million tonnes, putting it in second place as a source for China ahead of Saudia Arabia in third.

The report said it was the first time Russia has been China's top crude supplier since October 2005.

Russia has strengthened ties and trade with China since coming under Western sanctions for its actions in Ukraine.