Ship-to-ship trade with North Korea on the high seas is also forbidden under UN rules but very hard to patrol without an aggressive Chinese crackdown on smuggling.
Robert Kelly, professor of political science at South Korea's Pusan National University, ventured that the reports were entirely plausible. "There is a lot of under-the-radar on the Chinese side. Beijing does not police the border strictly or enforce the sanctions toughly. This could be that," he said.
Meanwhile analysts were surprised by the latest Chinese customs data released on Tuesday, showing that, in a rare move, Beijing exported no oil products to North Korea in November.
Chinese foreign ministry spokeswoman Hua Chunying said she didn't know any details about the oil products export situation, but added: "As a principle, China has consistently fully, correctly, conscientiously and strictly enforced relevant UN Security Council resolutions on North Korea."
It is unknown if China still sells crude oil to Pyongyang as Beijing has not disclosed its crude export data for several years.