U.S. reconnaissance satellites have spotted Chinese ships selling oil to North Korean vessels on the West Sea around 30 times since October.

According to South Korean government sources, the satellites have pictured large Chinese and North Korean ships illegally trading in oil in a part of the West Sea closer to China than South Korea.

The satellite pictures even show the names of the ships. A government source said, "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."

The U.S. Treasury Department placed six North Korean shipping and trading companies and 20 of their ships on sanctions list on Nov. 21, when it published spy satellite images taken on Oct. 19 showing a ship named Ryesonggang 1 connected to a Chinese vessel.