SEOUL, South Korea — More than 33,000 tons of North Korean coal was illegally imported to South Korea last year in violation of United Nations and other sanctions, the South Korean government said Friday.

A government customs agency, the Korea Customs Service, asked prosecutors to indict the heads of three South Korean companies that transport coal. It said the companies imported the North Korean coal, totaling $5.8 million in value, between April and October of last year, and it accused them of falsifying documents to indicate that the coal was of Russian origin.

The owners of 14 ships involved in transporting the coal from North Korea to ports in the Russian Far East, and from there to South Korea, could also face penalties under United Nations sanctions, South Korean officials said. It was unclear whether the owners of the ships that loaded the coal in Russia knew it was from North Korea.

Last August, the United Nations Security Council barred all member nations from importing North Korean coal, iron ore and other significant sources of hard currency for the isolated regime. Separately, South Korea has banned trade with North Korea since the North was accused of sinking a South Korean Navy ship in a torpedo attack in 2010.