More than 35,000 tons of North Korean coal and iron worth 6.6 billion won ($5.8 million) were imported into the South between April and October last year, the Korea Customs Service (KCS) said.
In addition to breaking South Korean law, some of the shipments were likely in breach of UN resolutions as well, with the customs service warning that "any ships that are believed to have violated UN sanctions will be impounded or banned from entering South Korean ports."
In a complex process spanning three countries, coal shipments were first sent to Russia, where their details were disguised using forged "country of origin" documents, and then reloaded on ships bound for the South, the customs office said that followed a 10-month investigation by the authorities.
In another case, black coal imported from North Korea was transferred to the Russian port of Kholmsk, where it was "disguised as semi-coking coal that does not need country of origin documents," before being reloaded on ships heading to the South, the KCS said.
"The KCS has confirmed seven criminal offences and it will report three persons and three companies to prosecution authorities with a request to indict them," it said.
"It is necessary to consider the timing of UN resolutions... to determine whether the activities were in breach of sanctions as well."
News of the apparent breach comes after a UN report last week accused the North of evading sanctions by continuing to export coal, iron and other commodities as well as carrying out illegal ship-to-ship transfers of oil products at sea.
Deliveries of iron and steel generated nearly $14 million from October to March, the UN report said.
The South Korean investigation also uncovered a trade in pig iron, with the customs office saying that the suspects sold the crude metal to a South Korean buyer via a Hong Kong-based shell company.