American reconnaissance satellites have reportedly spotted Chinese ships suspected of selling oil to North Korean vessels about 30 times since October.
South Korean officials told the Chosun Ilbo that the ships were allegedly trading in the West Sea between China and South Korea in a bid to bypass strict United Nations sanctions on oil exports to the pariah regime over its ongoing nuclear and weapons programme.
"We need to focus on the fact that the illicit trade started after a UN Security Council resolution in September drastically capped North Korea's imports of refined petroleum products," an unnamed source told the paper.
The US Treasury published surveillance photographs reportedly taken on October 19 of the North Korean vessel Rye Song Gang 1 lashed to a large Chinese vessel in deep waters, apparently showing hoses transferring oil.
Under the current tough sanctions regime Pyongyang is only allowed 500,000 barrels of oil imports a year. North Korea last week denounced ever-tightening economic sanctions as "an act of war".
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.