The Japanese government on Tuesday distanced itself from comments by a prominent politician that the so-called "comfort women" of WWII served a "necessary" role by keeping troops in check.
Outspoken Osaka Mayor Toru Hashimoto said soldiers living with the daily threat of death needed some way to let off steam which was provided by the comfort women system.
Up to 200,000 women from Korea, China, the Philippines and elsewhere were forcibly drafted into brothels catering to the Japanese military in territories occupied by Japan during WWII, according to many mainstream historians.
"When soldiers risk their lives under a hail of bullets, and you want to give them a rest somewhere, it is clear that you need a comfort women system," Hashimoto said.
South Korea voiced "deep disappointment" over the comments, which risk inflaming Japan's relationship with neighbours that were victims of brutal expansionism and who claim Tokyo has never faced up to its warmongering past.
"There is worldwide recognition... that the issue of comfort women amounts to a wartime rape committed by Japan during its past imperial period in a serious breach of human rights," a Seoul foreign ministry spokesman told AFP.
"Our government again urges Japan's prominent officials to show regret for atrocities committed during Japan's imperial period and to correct their anachronistic way of thinking and comments."
Hashimoto, who is co-leader of the national Japan Restoration Party, acknowledged that some women providing sexual services to Japan's soldiers did so "against their will", something he attributed to "the tragedy of war".
But he said there was no evidence this had been officially sanctioned by the state and that the use of prostitutes by servicemen was not unique to Japan.
"There are many examples" of unacceptable and brutal behaviour by soldiers in wartime and "to contain such things, it is a cold fact that a certain system like comfort women was necessary", he said.
Japan's top government spokesman and Chief Cabinet Secretary Yoshihide Suga on Tuesday refused to comment directly on Hashimoto's remarks.
However, he said: "The government's position on the comfort women issue is that, as I repeatedly said here, we feel pains towards people who experienced hardships that are beyond description and (this) administration shares the view held by past governments."
In a landmark 1993 statement, the Japanese government offered "sincere apologies" for the "immeasurable pain and suffering" inflicted on comfort women.
Two years later, Japan issued a broader apology expressing "deep remorse" for war suffering.
The 1993 statement remains passionately opposed by some Japanese conservatives who contend that the country did not directly coerce women.
Despite a hawkish stance on history, Prime Minister Shinzo Abe indicated last week he does not intend to backtrack on these apologies.
Japan's shared history with its Asian neighbours looms over present-day relations, which are also strained by separate territorial disputes with Seoul and Beijing.
Both capitals say Tokyo has not shown sufficient contrition for its WWII behaviour. But many in Japan feel nationalists abroad use the issue as a stick to beat it for their own domestic ends.
Hashimoto, who was once mentioned as a possible future prime minister, said Monday that Japan bears responsibility for the war and urged compassion for victims.
"(Comfort women were) a result of the tragedy of war so we have to take care with thoughtfulness of those people who became comfort women against their will," he said.
Shintaro Ishihara, a former Tokyo governor and the other co-leader of the Japan Restoration Party, came to Hashimoto's defence on Tuesday, arguing prostitutes and militaries have co-existed throughout history.
"Although Mr Hashimoto's comments are unpleasant to hear, he is not saying anything wrong," he said.