South Korea's intelligence chief has met Japanese Prime Minister Yoshihide Suga and called for efforts to improve relations between the two countries strained over a string of historical issues, a diplomatic source said Thursday.

Park Jie-won, the head of the National Intelligence Service (NIS), paid a courtesy call on Suga on Wednesday as he was visiting Tokyo to attend a trilateral meeting with his Japanese and US counterparts.

At the meeting, Park said that relations between the Asian neighbors "should not remain like this," saying efforts are necessary to improve ties, and Suga echoed the need, the source said without providing more details.

Bilateral ties between South Korea and Japan remain frayed over the prolonged row regarding wartime forced labor and sexual slavery issues.

Park previously met the Japanese prime minister in November last year, when he also delivered South Korean President Moon Jae-in's willingness to normalize relations.

Earlier on the same day, Park reportedly met with Hiroaki Takizawa, Japan's cabinet intelligence director, and US Director of National Intelligence Avril Haines at an undisclosed place in Tokyo and discussed North Korea and other issues.

He held a separate meeting with Takizawa on Tuesday and exchanged views on bilateral issues, local media reports said.

Seoul's NIS declined to comment on his trip to Japan and media reports on those meetings. (Yonhap)