US satellites have spotted Chinese tankers transferring oil to North Korean ships 30 times in three months – despite strict UN trade embargoes, according to reports.

Overhead images appear to show ships from the two countries shackled together for a fuel transfer in the West Sea off China.

Such ship-to-ship trades are banned under a UN Security Council resolution adopted in September, but according to South Korean government sources, American satellites have pictured large vessels from both China and North Korea illegally trading in a stretch of the West Sea on multiple occasions.

One picture, reportedly taken on October 19, shows a ship called Ryesonggang 1 connected to a Chinese vessel, South Korean media reports. The US Treasury Department later placed six North Korean shipping and trading companies and 20 of their vessels on sanctions list. It said the activity appeared to show attempts to bypass sanctions, though it has not been suggested that Chinese authorities were aware of the transactions.

It comes a day after Chinese customs data was revealed claiming Beijing exported no oil products to North Korea in November. The figures apparently go above and beyond sanctions imposed earlier this year by the United Nations in a bid to limit petroleum shipments to the isolated country.

Tensions have flared anew over North Korea’s ongoing nuclear and missile programmes, pursued in defiance of years of U.N. resolutions. Last week, the U.N. Security Council imposed new caps on trade with North Korea, including limiting oil product shipments to just 500,000 barrels a year. Beijing also imported no iron ore, coal or lead from North Korea in November, the second full month of the latest trade sanctions imposed by U.N.

China, the main source of North Korea’s fuel, did not export any gasoline, jet fuel, diesel or fuel oil to its isolated neighbour last month, data from the General Administration of Customs showed on Tuesday.