His idea came as Japan has rejected Seoul's so-called one-plus-one proposal in June that South Korean and Japanese firms jointly create a fund to compensate Korean victims of Tokyo's wartime forced labor.

"(My idea) is to add voluntary contribution by people of the two countries to the proposed fund, on top of companies of both nations," Moon said at Waseda University.

Tokyo has reacted angrily to the South Korean top court's 2018 rulings that ordered Japanese firms to compensate the Korean victims. It claims all reparation issues related to Japan's 1910-45 colonization of Korea were settled in a 1965 state-to-state accord on normalizing diplomatic ties.

Moon also suggested the use of an outstanding 6 billion won ($5.2 million) that was left over from a now-dissolved fund to resolve the issue of Japan's wartime sex slavery.

The Korean government has shut down a 1 billion-yen foundation on Japan's wartime sex slavery that was funded by Japan. The fund was created as the outcome of a 2015 deal with Japan under conservative President Park Geun-hye.

But it remains uncertain whether Moon's idea could be a viable option, given a sharp difference over the two nations' stances on the issue.

The speaker arrived in Tokyo on Sunday for a four-day visit to take part in a parliamentary speaker's meeting of the Group of 20 advanced countries. (Yonhap)