South Korean Prime Minister Lee Wan-koo issued a strong warning against Japan Thursday for its continued attempt to distort the countries' shared history.
  

In a press conference, Lee warned that the Japanese government will face "grim historical judgment" if it sticks to such a wrongful campaign.
  

He stressed Japan "can't cover up unequivocal historical truth."
  

It is highly unusual for South Korea's prime minister to directly deliver such a tough message against a neighboring country and major economic partner.
  

It came in response to Japan's assertion that its ancient government set up a military outpost in the southern part of Korea in the fourth century and ruled a nation there at that time.
  

In a separate statement, Seoul's foreign ministry expressed regret over Japan's assertion, dismissing it as groundless.
  

"It's my understanding that even Japan's academic circles commonly believe the theory is groundless," the ministry's spokesman Noh Kwang-il said at a press briefing. "(The government) will call again on Japan to correct relevant descriptions."   
  

The Shinzo Abe administration also intensified its claim to Dokdo, a pair of Seoul-controlled islets in the East Sea, in new school textbooks and an annual foreign policy paper earlier this week.
  

Abe plans to deliver a speech at the joint session of the U.S. Senate and the House of Representatives on April 29, which is expected to include his perception on the historical issues.
  

The South's prime minister instructed the nation's related government agencies to step up research of historical facts.
 

Japan colonized the peninsula from 1910-45. Many Koreans think Japan has yet to offer a sincere apology for its wartime atrocities. (Yonhap)