Following the US takeover of the Venezuelan oil industry, commodities trading firms were given contracts to market the crude to buyers across the world, including to China.

But Venezuelan crude oil is now being sold at far higher prices than before, with the profits routed through US companies and energy traders.

The higher prices have pushed Chinese refiners out of the market for the heavy crude from Venezuela, and they are shifting their orders to Canada, Russia, and Iran.

Canadian tar sands oil is more expensive than Venezuelan heavy sour, but is similar, and offers far shorter transit times and lower shipping costs.

Chinese energy traders have been instructed to refuse new offers for Venezuelan crude.