Japan is protesting South Korea’s indictment of a Japanese journalist for allegedly defaming South Korean President Park Geun-hye.
The charges against Tatsuya Kato relate to an article in the Sankei Shimbun that alleged Park did not respond quickly enough to April's ferry disaster that killed over 300 students.
The report repeated accusations from South Korean media that the president was away from her office for several hours on the day of the sinking, possibly because she was with a man.
South Korean prosecutors say Kato failed to adequately verify his report, which they say contained falsehoods. Charges have not been filed against South Korean outlets that reported the rumor.
Prison sentence possible
If found guilty, Kato faces up to seven years in prison. Although he has not been arrested, the 48-year-old journalist has been prevented from leaving South Korea.
On Thursday, Japan summoned a South Korean envoy to express Tokyo's "deep concern" about the indictment. Chief Cabinet Secretary Yoshihide Suga said the move is "extremely regrettable" for freedom of the press and bilateral relations.
Japan-South Korea ties are already strained over a long-standing territorial dispute, as well as over what many South Koreans see as Japan's failure to atone for its abuse of South Korean sex slaves during World War II.
The frosty relations prevented President Park from meeting one-on-one with Japanese Prime Minister Shinzo Abe.
Suga on Thursday said his government would still like to arrange such a summit, particularly because the two countries have disagreements.