Three South Korean companies illicitly purchased and imported North Korean coal disguised as Russian product in a flagrant violation of U.N. sanctions, the Korea Customs Service announced on Friday.

After examining nine separate cases of illicit imports from the North, the agency determined around 35,000 tons of coal worth $5.8 million was shipped to South Korea between April and October in 2017, in what would have been a sizeable boost for the North Korean economy.

“The firms appear to have illicitly brought it in expecting big trading margins after prices of North Korean coal had dropped due to the import ban,” Roh Suk-hwan, deputy commissioner of the Korea Customs Service, said at a news conference on Friday.

Such imports are a flagrant violation of Security Council resolution 2371 adopted last August that prohibits the export of North Korean minerals such as coal, a move aimed at the country’s economy in response to its continued nuclear weapons development.

Meanwhile, the South Korean customs agency confirmed it would press charges against both companies, and also involved individuals for violating sanctions and for the forgery of private documents.

The shipments are merely the latest case of North Korea bypassing international sanctions on export markets. Last month, it emerged that Russia had illegally sent North Korean coal to South Korea on two separate occasions, with shipments worth hundreds of thousands of dollars from the Russian port of Kholmsk arriving in the South Korean ports of Incheon and Pohang.

“Our government is making diplomatic efforts necessary to implement the U.N. Security Council sanctions on North Korea in close coordination with the international community,” ministry spokesperson Noh Kyu-duk said at the time.

In January, U.S. officials presented satellite photographs showing Chinese cargo ships loading North Korean coal in violation of U.N. sanctions. A report from the Wall Street Journal detailed how six Chinese vessels tracked by American satellites attempted to avoid detection by turning off their automatic identification systems as they approached North Korea waters to pick up their illicit loads of coal.

This occurred despite Beijing declaring their own ban on North Korean products such as coal, iron, iron ore, and seafood in response to the regime’s aggressive nuclear expansion. China represents around 90 percent of the North’s total trading activity.

North Korea’s coal reserves remain a fundamental part of the communist country’s economy, with some estimates suggesting it has around 100 billion metric tonnes in coal and limestone worth up to $9.7 trillion.

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