Prime Minister Lee Wan-koo on Thursday urged Japan not to distort facts related to the history between the two countries, days after Tokyo stepped up its territorial claims to the easternmost islets of Dokdo.

“(Japan) must refrain from distorting historical facts based on things that are simply not true ... History will sternly judge such attempts to cover up the truth,” Lee said in a meeting with reporters at the Seoul Government Complex in central Seoul.

Lee specifically criticized Japan’s so-called theory of Imna Nihon-fu, which claims that Japan ruled the southern parts of the Korean Peninsula from the 4th to 6th century, with Japan’s Culture Ministry website and several state-approved textbooks recently presenting content related to this theory.

Lee expressed regret that the theory was still being used, despite the two countries reaching an agreement in 2010 not to do so, and said the government needs to provide more support for historical research.

The unscheduled press conference by the nation’s second-in-command came in response to Japan reasserting its sovereignty over Dokdo in an annual diplomatic paper this week. It followed Tokyo’s approval of a batch of updated middle school textbooks carrying stronger claims on the islets earlier in the week.

The series of moves soured the relationship between the two countries, which has long been hampered by the Shinzo Abe administration’s apparent attempt to whitewash the country’s atrocities during World War II, such as the sexual enslavement of Korean and other women.

The premier had previously called Abe’s perception of history “regretful,” and expressed concern that the younger generation of Japanese people may end up with an incorrect view of history.

In a separate briefing, South Korea’s Foreign Ministry expressed regrets about the inclusion of the Imna Nihon-fu theory in Japan’s textbooks. The ministry called the theory “groundless” and said the government plans to urge Japan to revise the parts related to it.

Lee stressed that he would “keep an eye” on the situation, saying that if Japan continued to try to distort historical facts, it would be “yet another problem.”

“The relationship between South Korean and Japan must be a healthy one with economic and security cooperation. But we will not permit any distortion of history in any circumstance, as such actions deny the spirit of our people,” he said.

By Yoon Min-sik (minsikyoon@heraldcorp.com)