A South Korean court on Friday ordered the Japanese government to pay compensation to 12 World War II sex slaves or their families, in an unprecedented ruling likely to infuriate Tokyo.

The Seoul Central District Court ruled that Japan should pay the victims 100 million won ($91,000) each, South Korea's Yonhap news agency reported.

It is the first civilian legal case in South Korea against Tokyo by wartime sex slaves for Japanese troops, who were euphemistically labelled "comfort women".

Tokyo and Seoul are both major US allies, democracies and market economies faced with an overbearing China and nuclear-armed North Korea.

But their relations are strained by Japan's early-20th century colonial rule over Korea, which is still bitterly resented on the peninsula, and have plunged to their worst in years under South Korea's centre-left President Moon Jae-in.

Mainstream historians say up to 200,000 women, mostly from Korea but also other parts of Asia including China, were forced to work in Japanese military brothels during World War II.

Friday's ruling came in a legal process that began eight years ago and several of the original plaintiffs have since died, to be replaced by family members.

Tokyo boycotted the proceedings and insists all compensation issues stemming from its colonial rule were settled in a 1965 treaty and linked agreement normalising diplomatic relations between the neighbours.

Under them, Japan paid South Korea financial reparations -- which Seoul used to contribute to its transformation into an economic powerhouse -- and the document said that claims between the states and their nationals had been "settled completely and finally".

The Japanese government denies it is directly responsible for the wartime abuses, insisting that the victims were recruited by civilians and that the military brothels were commercially operated.

The dispute has festered despite the treaty and Seoul and Tokyo reached a deal in 2015 aimed at "finally and irreversibly" resolving it with a Japanese apology and the formation of a 1 billion yen fund for survivors.

But Moon's government declared the agreement reached under his conservative predecessor faulty and effectively nullified it, citing the lack of victims' consent.

The move led to a bitter diplomatic dispute that spread to affect trade and security ties.

The same court is due to rule next week on a similar case brought against Tokyo by another 20 women and their families.

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