TOKYO (AFP) - Activists said on Monday that they had a trove of documents proving the Japanese military was complicit in the wartime system of sex slavery, despite nationalist quibbles over responsibility.
The Asian Solidarity Conference said it had more than 500 papers - many of them from the government - that supported the 1993 apology issued by Tokyo, in which it said it was sorry for the coercive brothel system.
Historians say as many as 200,000 Asian women, many of them from the Korean peninsula, were forced to work in military brothels catering to Japanese troops during World War II.
Despite the apology to the so-called "comfort women" and a 1965 deal normalising ties with Seoul and intended to settle all outstanding claims for wartime wrongs, the issue continues to mar relations.
While many Japanese accept the country's guilt, some senior politicians on the right - including Prime Minister Shinzo Abe - have suggested that the issue is overblown, and say there is no reliable evidence of official involvement or complicity in the sex slavery system.
Last week, Mr Abe's government said it would set up an expert team to review how the 1993 statement was drawn up and the historical facts it was based on.
The conference said Tokyo needed to be open and honest about its guilt and there should be no backtracking on the 1993 apology, called the Kono Statement.
"We hereby submit 529 items of official documents, researched and discovered by academics and civilians at home and abroad to further expose the facts behind the Japanese imperial military's 'comfort women' system," the group said in a letter addressed to the conservative prime minister.
The documents discovered over the past two decades include one that indicates wartime brothels were maintained as official military facilities, it said. Another document showed a chief military police officer used his official budget to cover up the fact that he had recruited "comfort women", it said.
The activist group urged Mr Abe's government to admit that the women were forced to provide sex against their will and that Japan make an apology in a "clear and official way that would be irreversible".
Five former comfort women - from South Korea, the Philippines and Indonesia - and the daughter of another from China, appeared at a press conference after the Monday meeting, calling on the Japanese government to officially and sincerely atone for the wartime sexual slavery.
Despite the establishment of the review, Mr Abe's top spokesman Yoshihide Suga has ruled out the possibility of revising the apology. It is unclear what will happen if the probe's findings are at odds with the 1993 statement, which was largely based on testimony from 16 Korean former comfort women, many of whom have since died.