South Korea has banned the port entry of four ships confirmed to have imported North Korean coal into the country.

A Foreign Ministry official said Sunday that the government had prohibited four ships that were used to transport resources banned under UN Security Council Resolution 2371 from entering local ports from Saturday. 

The four ships -- the Sky Angel, the Rich Glory, the Shining Rich and the Jin Long -- were confirmed to have transported North Korean coal into South Korea last year in violation of the 2017 resolution, which bans the North from exporting coal, iron ore and other minerals.

The ministry official said that the government plans to report its probe results on the alleged illegal shipments to the UN Security Council's sanctions committee as early as this week, adding the U.S. has already been briefed on the matter.

Last week, Seoul's customs office announced that three South Korean firms imported North Korean coal and pig iron from Russia on seven occasions between April and October last year in an apparent violation of the UN sanctions resolution.