SEOUL, South Korea — South Korea said a total of 35,000 tons of North Korean coal and pig iron worth $5.8 million illegally entered its ports last year, in possible violations of U.N. sanctions.

Reporting on preliminary results from a 10-month investigation, the Korea Customs Service said Friday it is seeking prosecutions of three local companies and their executives for smuggling or forging documents to say North Korean mineral resources came from Russia.

They imported North Korean coal or pig iron in seven separate cases between April and October last year to five South Korean ports, on the Jin Ao, Rich Vigor, Shining Rich and other vessels, the customs office said.

The coal originated from the North Korean ports of Wonsan, Chongjin, Daean and Songlim and were transshipped via the Russian ports of Kholmsk and Vladivostok.

Officials said they were also looking into whether any of the 14 vessels that transported North Korean coal violated sanctions banning such shipments. The United Nations banned North Korean mineral exports, including coal, starting in August 2017. Sales of its mineral resources provide a cash mainstay for North Korea. The bulk of revenue from those exports go to state-owned companies and help finance development of its missiles.

The finding comes as South Korean President Moon Jae-in’s administration pursues detente with the North. Hopes are high for economic cooperation and investment in North Korea once sanctions are lifted. Despite burgeoning diplomatic efforts to disarm North Korea, the international community has maintained maximum pressure on the North. North Korea has chafed at U.S. insistence that no sanctions be eased until Pyongyang disarms its nuclear weapons.

South Korea started looking into allegations of North Korean coal imports back in October and the government was criticized over how long it was taking to investigate. The customs officials said analyzing a huge volume of documents and seeking help from Russian customs officials slowed progress in the investigation.

Determining if sanctions banning exports or North Korean mineral resources were violated may take time.

“In order to sanction the vessels, we need reasonable evidence that they were involved in the activities banned by the U.N. Security Council,” Roh Suk-hwan, deputy commissioner of Korea Customs Service, said in a televised press conference. “We will likely discuss the timing of the UN Security Council’s resolutions against North Korea, the nationality of ships and other issues.”

The Security Council first imposed sanctions on North Korea after its first nuclear test in 2006. A recent report to the Security Council found Pyongyang has been violating U.N. sanctions with clandestine shipments of coal, oil and military equipment.