An official at the South Korean Justice Ministry said he was aware the mother had taken up the issue of the alleged human rights abuses with both the Japanese government and the South Korean Embassy in Tokyo. "But we have yet to receive any information about the human rights abuse case from the embassy," the official said.

The ministry is seeking the man's transfer to South Korean custody in accordance with an international prisoner transfer treaty, the official went on, adding, "We will try to have him transferred from the Japanese prison as part of efforts to protect Korean nationals abroad."

In February, the Tokyo High Court upheld a lower court ruling that sentenced the man to four years in prison for the detonation of the bomb in a public restroom at the Yasukuni Shrine.

The 29-year-old suspect, identified only by his surname Chon, was found guilty in July last year by the Tokyo District Court of breaking into the premises of the shrine on Nov. 23, 2015, and setting up the bomb in a public restroom.

The incident, which resulted in no loss of life but damaged the restroom's ceiling, prompted a bomb disposal unit to be mobilized.

He was arrested in December 2015 shortly after voluntarily returning to Tokyo by plane.

In the February ruling, the court also found Chon guilty of violating laws on explosives control and tariffs.

Chon and his legal representative have been demanding a suspended sentence, as he had no intention of hurting people and admitted to the wrongdoings. Prosecutors had originally sought a five-year sentence.

South Korea and China view the Yasukuni Shrine -- which honors over 2.4 million war dead, including 14 convicted Class-A war criminals -- as a symbol of Japan's past imperialism.

Many South Koreans still harbor deep resentment against Japan over its 1910-45 colonial rule of the Korean Peninsula. (Yonhap)