The head of the South Korean National Assembly's foreign affairs committee has sent letters to UNESCO panel members expressing opposition to Japan's bid to register a group of 23 industrial sites for World Heritage status, a source close to the parliamentarian said Wednesday.

Na Kyung-won, chairwoman of the Foreign Affairs and Unification Committee, noted in the letters to World Heritage Committee member countries that at seven of the 23 sites around 57,900 Koreans were conscripted to work under harsh conditions while Korea was under Japanese rule, according to Yonhap News Agency.

"Space and time continue. Japan is unable to deny (forced labor) existed, so it is trying to avoid it," a South Korean foreign ministry official told a small group of Western media.