The three entities were found to have transshipped the resources at a Russian port and misstated the country of origin to bring them into South Korea, Seoul officials said.

The investigation came amid media speculation that Seoul has not been taking due steps to block the entry of the illegal North Korean products as party of efforts to denuclearize the North and foster a lasting peace on the peninsula.

The North is banned from exporting coal, iron ore and other mineral resources under Resolution 2371 adopted in August last year. UN sanctions also call for a country to capture and look into a vessel suspected of engaging in prohibited activities with the North.

Meanwhile, Seoul's Vice Foreign Minister Cho Hyun ruled out the possibility that South Korean firms, which unknowingly purchased North Korean coal, would be subject to US "secondary boycott" measures that penalize entities doing business with the North.

"The firms just bought such coal through the transparent bidding procedures," Cho said in an interview with Voice of America. "Those subject to punishment are the individuals that caused the problem."

Asked about the criticism of a drawn-out probe into the illegal shipments, Cho pointed out that Seoul sought to "accurately" investigate the case.

"In accordance with the principle of being presumed innocent until proven guilty, (we) could not inform (the public of the investigation content) in the interim period, and it took time as (we) tried to accurately probe it," he said.  (Yonhap)