A wartime letter written by a Japanese national has confirmed that Japan had "forced" Korean women to become sex slaves for the Japanese army during World War II, in a rare documentary evidence that could prove Tokyo's culpability for its wartime atrocities, according to the letter released by China on Monday.

Historians say up to 200,000 women from Korea, China and some Asian nations were coerced into sexual servitude at front-line Japanese brothels during the war. Those sex slaves were euphemistically called "comfort women."

According to one of the Japanese-language letters revealed by China's Jilin Provincial Archives to some South Korean media organizations, Japan's government "forced" Korean women to serve as sex slaves for Japanese soldiers who were stationed in northern China.

The letter, written by a Japanese citizen named Nakata who lived in Heilongjiang province of China in 1941, said, "Under the National Mobilization Law, some 20 Korean women were forced to serve at a comfort station in the Japanese army compound." It was referring to the Japanese law legislated in 1938 to put Japan's economy on wartime footing after the start of the Second Sino-Japanese War.

Japan's military gave "pink-colored ration tickets" to the victims of sexual slavery as "payments," the letter said.

The letter was not sent because it was censored by the Japanese army's "Monthly Postal Review Report." It has been preserved at the Jilin archives administration, which keeps 100,000 pieces of Japan's wartime documents and analyzes them.

Zhao Yujie, a researcher at the Jilin archives administration, noted that it was a rare evidence that Japan mobilized Korean women as sex slaves under the 1938 law.

"It can be recognized that the Korean women were forcefully mobilized as comfort women by the Japanese law," Zhao said. "Also, the term of 'forced mobilization' was shown in the letter."

China's foreign ministry took journalists of some South Korean news organizations, including Yonhap News Agency, to the Jilin archive administration and revealed 25 pieces of the documents, including the letter.

One of the documents written by the Japanese military police in Nanjing during the war showed that it operated a military brothel called "comfort station" in Wuhu, a county of Anhui province.

The Japanese document described 36 Korean women at the military brothel as "special comfort women," strongly indicating that they were coerced into sexual servitude.

Yin Huai, head of the Jilin archives administration, told reporters that China hopes to work together with South Korea to discover more evidence of wartime crimes committed by the Japanese army during the war.

"Korea is a close neighbor of China and shared sufferings during the war," Yin said. "We want to share this finding with relevant Korean sides and develop this research further."

Relations between South Korea and Japan have been frayed in recent years due to various disputes over their shared history. In recent weeks, tensions escalated over revelations that the Japanese government may re-examine the apology made to the former sex slaves.

Last week, however, Japanese Prime Minister Shinzo Abe dismissed concerns that the re-examination is aimed at retracting the apology, saying he has no plans to revise the statement issued by then Chief Cabinet Secretary Yohei Kono in 1993.

The so-called "Kono Statement" acknowledges that the Imperial Japanese Army was involved to some extent in the recruitment of women to serve in front-line brothels.

South Korea has pressed Japan to address long-running grievances by the victims of wartime sex slavery by extending a formal apology and providing compensation to them. But Japan has refused to do so, saying the matter was settled by a 1965 treaty that normalized relations between the two countries.

Time is running out for those aging victims in South Korea, with only 55 remaining alive today. Their average age is 88. (Yonhap)