South Korea said Friday three South Korean companies apparently violated United Nations sanctions by importing nearly 35,000 tons of coal and iron worth nearly $6 million from the North last year.

The Korea Customs Service disclosed the results of a 10-month investigation into the shipments Friday and said it will prosecute executives of the company for smuggling or forging documents to say the North Korean resources came from Russia.

The report says coal or pig iron was imported seven times between April and October last year to five South Korean ports.

The process involved three countries. North Korean coal shipments were first sent to Russia, then reloaded on ships bound for South Korea.

The United Nations banned exports of North Korean coal and other minerals in August 2017. The sale of North Korean minerals generates revenue that supports state-owned businesses and helps finance its missile program.

The report was released as Seoul pursues closer relations with North Korea, with hopes of economic cooperation and investment in North Korea once sanctions against Pyongyang are lifted.

The report comes after a U.N. report last week accused the North of avoiding sanctions by continuing to export coal, iron and other commodities and illegally transferring oil products from ship-to-ship.

Greater diplomatic efforts to disarm North Korea are underway while the global community maintains maximum pressure on the North. Pyongyang has bristled at U.S. demands that sanctions remain in effect until it dismantles its nuclear weapons.