Last year, Rina Gonoi went public about the assaults she was subjected to, after an investigation was dropped on the grounds of insufficient evidence.

The military subsequently acknowledged the assaults and harassment that occurred in 2021, but only after public pressure by Gonoi, including a petition signed by over 100,000 people demanding an investigation.

In December, five Japanese soldiers were dismissed over the case, but Gonoi said Monday the apologies she has received were "superficial" and lawyers for the assailants continued to trivialise the incidents in discussions over a settlement.

"I didn't want to choose the option of fighting, but I have not received a message that reassures me they really regret" what they did, she told reporters.

"Given the differences over the issue between their side and me, I think it's necessary to open things up (to the public through the lawsuit), which I think will help prevent a repeat," she added.

Gonoi is seeking a total of 7.5 million yen ($58,000) -- 5.5 million from her attackers for mental distress, and two million from the government for failing to prevent the assaults and properly investigate them.

Her case is also being re-investigated by prosecutors who are weighing possible criminal charges after she lodged a complaint about the failure to indict the men involved, her lawyers said Monday.

The army acknowledged its probe found that Gonoi routinely faced sexual harassment and assault at her unit and during training sessions.

While Japan may rank high on education and healthcare for women, its male-dominated society has long lagged behind industrial peers in placing women in boardrooms and in high public office.

Government data shows just four percent of rape victims report the crime to police.