Japan said on Tuesday it can "by no means" accept South Korea's call for more steps to help "comfort women," a euphemism for girls and women forced to work in Japan's wartime brothels, a divisive issue that Japan says was resolved with a 2015 deal.
Japan and South Korea share a bitter history that includes Japan's 1910-45 colonisation but the US allies are central to efforts to rein in North Korea's nuclear and missile programs.
A South Korean investigation appointed by the government concluded last month the dispute over the women could not be "fundamentally resolved" because the victims' demand for legal compensation had not been met.
Japan responded by saying any attempt by South Korea to revise the 2015 deal, struck by a conservative South Korean government, would make relations "unmanageable."
South Korea would not seek to renegotiate the 2015 agreement, even though it failed to meet victims' needs and resolve the feud, Foreign Minister Kang Kyung-wha said on Tuesday.
But she added that she hoped Japan would make further efforts to help the women "regain honour and dignity and heal wounds in their hearts."
Japan rejected any suggestion it take steps to augment the 2015 deal, under which Japan apologised to the victims and provided $8.8 million to a fund to support them.
"We can by no means accept South Korea's demands for additional measures," Japanese Foreign Minister Taro Kono told reporters in Tokyo.
Kono said the 2015 settlement was a "final and irreversible resolution."
"At a time when we are confronting the threat from North Korea, the Japan-South Korean agreement is an indispensable base for Japan-South Korean cooperation in various fields and the creation of a future-oriented relationship," Kono said.
South Korea's Kang did not elaborate on what additional steps South Korea was seeking but a senior foreign ministry official told Reuters it wanted a "voluntary, sincere apology rather than money".
South Korean activists estimate there may have been as many as 200,000 Korean women forced to work in the brothels.
There are 31 surviving women registered with the government, a foreign ministry official said.
In 2015, when there were 47, 36 accepted the settlement, the official said. But others rejected it, and some have been protesting regularly in front of the Japanese Embassy in Seoul.
Those opposed to the 2015 deal want Japan to take legal responsibility and provide due compensation.
South Korean President Moon Jae-in won an election last year after promises on the campaign trail to renegotiate the deal.
But he has also vowed to "normalise" ties with Japan.
The comfort women issue has been a regular cause for contention between Japan and neighbours China and North and South Korea since the war.
Japan colonised the Korean peninsula between 1910 and 1945 and occupied parts of China before and after the war.
Japan says the matter of compensation for the women was settled under a 1965 treaty. It says that in 2015, it agreed to provide the funds to help them heal "psychological wounds".