At 93, Kim Bok-dong died as she had lived for many years: at the heart of the controversy over Japan's use of forced labour in its wartime brothels.

Kim, who died on Monday at a hospital in the South Korean capital of Seoul, was a fixture at weekly protests outside the Japanese embassy calling for a sincere apology and compensation.

She remained angry at Japan until the end, her supporters said.

"She suddenly opened her eyes yesterday and told a long story... I couldn't decipher everything but one thing I could hear clearly was that we had to fight until the end," said Yoon Mee-hyang, who leads an advocacy group for the women.

"Then she expressed strong anger toward Japan as she continued talking, before she regained her tranquillity."

Painful past

Kim was among the two dozen known surviving South Korean "comfort women", a Japanese euphemism for women forced into prostitution and sexually abused at Japanese military brothels before and during World War II.

Activists say they plan to march alongside Kim's coffin past Japan's embassy on Friday, a demonstration that could further strain ties between Seoul and Tokyo.

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The two Asian neighbours share a bitter history stemming from Japan's 1910-45 colonisation of the Korean Peninsula.

Kim was 14 when she was first sent to a military brothel and forced to provide sex for Japanese soldiers in China, Hong Kong, Indonesia, Malaysia and Singapore, Yoon said. She became one of the first victims to come forward in 1992.

In 2012, Kim and another former comfort woman, Gil Won-ok, established the Butterfly Fund to help victims of sexual violence in armed conflicts around the world. National media reported Kim donated her life savings, 50 million won (US$44,762) to the organisation.

A sincere apology

Articulate and charismatic, Kim was a vocal critic of a 2015 deal in which Tokyo apologised to the victims and provided $9.1m to a fund in Seoul to help them.

Kim said it was not sincere because some Japanese leaders continued to deny the women were forced to work in brothels.

"We won't accept it even if Japan gives 10 billion yen. It's not about money. They're still saying we went there because we wanted to," Kim told a parliamentary session in 2016.

South Korean President Moon Jae-in's government has said it will not seek to renegotiate the 2015 deal. Last year, it vowed to shut down the Japan-sponsored fund and pursue a more "victim-oriented" approach.

Japan says the claims have been settled by past agreements and apologies, and the continued dispute may threaten relations between the two countries.

Just hours before Kim passed away, a fellow victim only identifiable by her surname Lee died, Yoon said.

Only 23 registered South Korean survivors are still alive, highlighting a sense of urgency behind efforts by the women to receive a formal apology and legal compensation from Japan while their voices can still be heard.