Japan on Thursday flatly rejected as "unacceptable" a South Korean proposal to set up a joint fund to compensate wartime forced labourers, a bitter dispute that has bogged down bilateral ties.

The proposed fund would not solve the issue, the top government spokesperson told a regular press conference in Tokyo.

"The South Korean proposal is utterly unacceptable," Chief Cabinet Secretary Yoshihide Suga told reporters.

"We will continue to urge South Korea to agree to arbitration," he added.

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Relations between the two US allies have been increasingly strained by a series of rulings from South Korean courts ordering Japanese firms that used wartime forced labour to compensate victims.

Japan's government and the firms involved have rejected the rulings, with Tokyo saying the issue was settled when the two countries normalised relations.

Agreement

Last month, Tokyo proposed the issue be put to arbitration under the terms of an agreement signed by the two countries in 1965, when ties were normalised.

The agreement calls for the two countries to set up an arbitration panel if they cannot resolve a dispute through diplomatic negotiations.

South Korea on Wednesday offered a counterproposal that would see South Korean and Japanese firms - including those involved in the court cases - set up a voluntary fund to compensate the victims.

"If Tokyo accepts our offer, our government is willing to review the Japanese government's request (to discuss the issue)," Seoul said in a statement.

Japan and South Korea are both democracies, market economies and US allies, but their relationship has been strained for decades as a result of Tokyo's brutal 1910 - 1945 colonial rule over the Korean peninsula.

The tense ties between Tokyo and Seoul come amid diplomatic efforts to persuade North Korean leader Kim Jong Un to give up his nuclear weapons, to ease one of the key security risks for the region.

When relations were normalised, Tokyo agreed a reparations package that included grants and cheap loans intended to cover victims of various wartime policies.

Japan argues that package should have permanently resolved the issue.