Last year, China replaced the US as the No 1 importer of oil from Venezuela, yet another front in the heated rivalry between Washington and Beijing.

The US had imposed sanctions on Venezuela's state-owned oil company as part of a bid to topple that country's socialist president, Nicolas Maduro. US refineries stopped buying Venezuelan crude. Caracas' ally China, long a major customer, suddenly found itself the top purchaser. Through the first six months of 2019, it imported an average of 350,000 barrels per day of crude from Venezuela.

But in August, Washington tightened its sanctions on Venezuela, warning that any foreign entity that continued to do business with the South American country's government could find itself subject to sanctions. State-owned China National Petroleum Corp, known as CNPC, stopped loading oil at Venezuelan ports that month. China's import data showed purchases started to slow, and by late 2019, abruptly stopped.

China's largest oil company, like customers in some other countries, seemed to be knuckling under to US President Donald Trump's threats, despite Chinese President Xi Jinping's professed support for Maduro.

But China never stopped buying. Crude from Petroleos de Venezuela SA, or PDVSA, kept arriving at Chinese ports with the help of a Switzerland-based unit of Rosneft, Russia's state-owned oil company, and a roundabout delivery method that made it appear as if the oil's origin was Malaysia, Reuters has found.

Between July 1 and Dec. 31, tanker ships delivered at least 18 shipments totalling 19.7 million barrels of rebranded Venezuelan crude to Chinese ports, Reuters determined. That finding is based on a review of ship-tracking data, internal PDVSA documents and interviews with four petroleum analysts who have tracked flows of Venezuelan oil around the globe.

A unit of CNPC chartered at least one of those tankers, meaning it was responsible for the oil aboard, the ship-tracking data show. That vessel, called the Adventure, took on Venezuelan crude on July 18 and discharged it in China on Sept 4, the data show. No charter information was available for the other ships that offloaded crude in China.