The unflagged cargo ship DE YI is docked in waters near the city of Busan since June 20. (Yonhap)

South Korean authorities have concluded that a detained cargo ship was carrying 4,500 tons of North Korean-produced anthracite coal in breach of UN sanctions, the Foreign Ministry in Seoul announced on Thursday.

The unflagged cargo ship DE YI has been docked in South Korean waters since March 30 for investigation, following intelligence obtained by the South Korean government implicating the vessel in actions contravening UN Security Council resolutions.

"Our government has decided to take the measure of detaining the vessel DE YI based on the results of the joint government investigation," a senior official at the Foreign Ministry, speaking on condition of anonymity, confirmed during a closed-door briefing.

The DE YI vessel received coal from the North Korea-flagged vessel Tok Song in March, in waters through a ship-to-ship transfer near Nampho Port in South Pyongan Province, the senior official explained.

The South Korean authorities have seen that the unflagged vessel was used primarily for the benefit of North Korea.

The DE YI vessel also notably departed from the port of Shidao in China on March 18 and navigated towards Nampho Port, turning off its automatic identification system to conceal its travel routes.

The vessel also loaded machinery, including electronic products, from the Chinese port and transferred them to another ship before receiving coal from the Tok Song vessel, The Korea Herald learned on background.

However, the South Korean investigation authorities have not reached a definitive conclusion regarding the destination of the North Korean coal, The Korea Herald also learned.

Satellite imagery captures the North Korean-flagged vessel Tok Song transferring North Korean-produced anthracite coal to the DE YI vessel via ship-to-ship transfer in the western waters near Nampho Port, South Pyongan Province, on March 21. (National Intelligence Service)

UN Security Council Resolution 2375 prohibits ship-to-ship transfers involving North Korea-flagged vessels, while UNSC Resolution 2371 bans the supply, sale and transfer of North Korea-produced coal.

However, controversy surrounds South Korea's decision to allow a Chinese national captain and most of the crew to leave, despite their involvement in violating UN Security Council resolutions on North Korea.

The unnamed Foreign Ministry official declined to provide details on the reasons behind the permit.

Instead, the official explained that the South Korean government imposed sanctions on Hong Kong-based HK Yilin Shipping Co Ltd. effective from Friday for possessing the DE YI vessel and "playing a pivotal role in actions violating UN Security Council resolutions."

Seoul also designated the Tok Song vessel for transferring North Korean-produced coal to the DE YI cargo ship, effective from Friday.

South Korea's Foreign Ministry explained that the Tok Song vessel is a used ship imported to North Korea in late March last year. Therefore, it also contravenes UNSC Resolution 2397, which prohibits the supply, sale and transfer of used vessels to North Korea.