SEOUL - South Korea's ageing victims of Japan's wartime sex slavery will file a US$20 million (S$26.8 million) lawsuit next month in the US to seek financial compensation from Tokyo.

The suit from 10 "comfort women" led by Ms Kim Bok Dong, 89, will be lodged at a California district court on July 1 against Japan, their attorney Kim Hyung Jin said yesterday.

The case, which is now being pursued in the United States because previous suits in Japan have failed, also targets Mitsubishi and other companies allegedly involved in war crimes.

Originally a dozen women who were forced to serve Japanese soldiers during World War II had offered to join the legal battle, but two have since died.

"The Japanese government should offer an official and sincere apology for wrongdoings by their ancestors and restore our honour," Ms Kim said outside the Japanese embassy in Seoul.

Mainstream historians say up to 200,000 women, mainly from Korea but also from China, Indonesia and other Asian nations, were forced into sexual slavery during the war.

Japan insists the issue was settled in the 1965 normalisation agreement, which saw Tokyo making a total payment of US$800 million in grants or loans to its former colony.

South Korea says Tokyo does not fully accept its guilt and has not sufficiently atoned.

The issue, which has remained in the background of diplomatic ties for the past few decades, has surfaced since Japanese Prime Minister Shinzo Abe and South Korean President Park Geun Hye came to power.

Ms Park and Mr Abe, however, gave a warmer impression in separate speeches on Monday as they marked the 50th anniversary of diplomatic relations between the two countries.

Ms Park previously said there can be no meeting with Mr Abe until Japan makes amends for its "comfort women" system.

But in a recent interview with the Washington Post, she said "there has been considerable progress on the issue of the comfort women", adding that the two countries were "in the final stage" of negotiations.

AGENCE FRANCE-PRESSE