South Korea lodged a strong protest Friday against Japan for laying a fresh territorial claim to its easternmost islets of Dokdo, warning it could further compromise their already strained relations.

Japan's "Diplomatic Bluebook for 2014" released earlier in the day identified the set of South Korean islets as Japanese territory, vowing to make efforts to resolve the territory issue through international law.

"South Korea expresses strong regret over the Japanese government's outrageous claim laid again through the 'Diplomatic Bluebook' on our own territory," Seoul's Ministry of Foreign Affairs said in a statement. "Such a claim will severely damage bilateral ties, as well as peace and security in Northeast Asia."

Japan's repeated claims to Dokdo show that the country is still under the spell of its history of imperialist invasions, the ministry said, adding that such nationalist moves will gravely impair the South Korea-Japan relations as well as the peace and security of the Northeast Asian region.

Seoul also denounced Japan's approval of revised local textbooks that contain more aggressive description of Japan's sovereignty claim to Dokdo. Japan was set to announce the approval on Friday, stepping up its territory claim.

"Our government strongly denounces Japan's approval of new elementary school textbooks, which stepped up provocative (remarks) on Dokdo, compared with those (approved) in 2010," the ministry said in a separate statement.

Only three weeks ago, Japan's Prime Minister Shinzo Abe pledged to "inherit" the apologetic historical perspective of previous cabinets. If Japan opts to teach its elementary students distorted history of its imperialistic past, that would amount to breaking its own promise made by Abe, the ministry said.

"Our government clearly warns Japan that the path for mending South Korea-Japan relations will become longer if the Japanese government continues its provocations regarding Dokdo in the name of textbook examination," the ministry said.

The foreign ministry is planning to call in Japanese Ambassador to South Korea Koro Bessho to the ministry headquarters Friday afternoon as part of Seoul's protest of the fresh Dokdo claims.

Combined with the unresolved issue of Japan's sexual enslavement of Korean women during World War II, the territory claims to Dokdo have severely soured bilateral relations.

Japan has renewed its claim to the set of rocky islets after its nationalist Prime Minister Shinzo Abe took power last year.

South Korea rejects Japan's territorial claims because Seoul regained its independence from Japanese colonial rule and reclaimed sovereignty over its territories, including Dokdo and many other islands around the Korean Peninsula. South Korea keeps a small police detachment on the islets.

Concerns have risen that the rekindled Dokdo issue may further fray already strained bilateral relations.

But experts said that the issue will not immediately inflame the bilateral situation, given that the Friday claims had long been predicted and that the two countries have begun the process to mend ties, including their on-going discussion to hold director general-level talks aimed at resolving the wartime sex slave issue.

Since taking office more than a year ago, President Park Geun-hye and Prime Minister Abe held their first summit meeting in The Hague last week as part of a three-nation summit that also included U.S. President Barack Obama.

Obama's late-April trip to Asia including Seoul and Tokyo is also expected to provide a breakthrough in the restrained bilateral ties with Washington hoping for better relations among its Asian allies due to the importance of the trilateral military partnership aimed at checking a rising China and nuclear-armed North Korea. (Yonhap)