Sanctioned Russia becomes China's main source of oil
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Beijing has refused to publicly condemn Moscow's war and has instead exacted economic gains from its isolated neighbour.
It imported 7.82 million tonnes of oil from Saudi Arabia in May.
China bought $7.47 billion worth of Russian energy products last month, about $1 billion more than in April, according to Bloomberg News.
The new customs data comes four months into the war in Ukraine, with buyers from the United States and Europe shunning Russian energy imports or pledging to slash them over the coming months.
According to the International Energy Agency's latest global oil report, India has overtaken Germany in the last two months as the second-largest importer of Russian crude.
Days before Moscow's invasion of Ukraine, China's President Xi Jinping greeted his Russian counterpart Vladimir Putin in Beijing where the two countries declared a bilateral relationship of "no limits".
Although demand in China remains muted due to Covid restrictions, there has been some improvement in the past month as cities loosen controls after the country's worst outbreak since the early days of the pandemic.
This has allowed supply chain problems to ease and industrial production to pick up, official data shows.
Beijing's purchases of Russian liquefied natural gas surged 54 percent on-year to 397,000 tonnes, even as overall imports of the fuel fell.
Once bitter Cold War rivals, Beijing and Moscow have stepped up cooperation in recent years as a counterbalance to what they see as US global dominance.
This month they unveiled the first road bridge linking the countries, connecting the far eastern Russian city of Blagoveshchensk with the northern Chinese city of Heihe.
Last week Xi assured Putin of China's support on Russian "sovereignty and security" in a call between the two leaders.
The Kremlin said the pair had agreed to ramp up economic cooperation in the face of "unlawful" Western sanctions.
The West has implemented unprecedented sanctions on Russia in retaliation for its war in Ukraine, forcing Moscow to find new markets and suppliers to replace foreign firms that have left Russia following the invasion.
The 27-nation European Union agreed in late May to a package of sanctions that would halt the majority of Russian oil imports.
The United States has already banned all Russian oil but European nations are much more dependent on these imports.
Energy is a major source of income for Putin's government, and Western nations are trying to isolate Moscow and impede its ability to continue the war.