South Korea on Thursday expressed "deep" concerns over Japan's repeated moves to deny its wartime atrocities as Tokyo's prime minister reportedly sent a message earlier this year to a ceremony to honor its war criminals.

In April, Japanese Prime Minister Shinzo Abe sent a condolence message to a memorial service held in a temple in western Japan to commemorate Japanese wartime leaders, including convicted class-A war criminals, according to Japanese media.

Seoul's foreign ministry said a series of Abe's nationalistic remarks and behaviors have raised doubts about whether Tokyo is sincere about facing up to its history.

"Seoul has no choice but to express deep concerns over Abe's recent remarks over war criminals that are seen as denying the post-war order and have also raised questions about the sincerity of the apology made by the Japanese government over its wartime atrocities and colonial rule," Noh Kwang-il, spokesman at the foreign ministry, told a press briefing.

Seoul's reaction came as the bilateral relations are at their lowest ebb in recent years due to Japan's attempts to gloss over its wartime atrocities such as sex slavery and its territorial claims to South Korea's easternmost islets of Dokdo.

Abe sent a ritual offering to the controversial Yasukuni war shrine through his aides on Aug. 15, the 69th anniversary of Japan's defeat in World War II, although he did not visit there.

The war shrine, which honors 14 class-A war criminals among other war dead, has been regarded as a symbol of Japan's past militarism.

Abe visited the war shrine in December, the first visit of its kind by a Japanese prime minister since 2006, sparking fierce criticism from Seoul and Beijing, which have a shared history. (Yonhap)