- Japan tried to register kamikaze pilots' wills with UNESCO in February
- Now China says they have registered records of 'comfort women' recruitment
- UNESCO's Memory of the World program protects important historical documents
- Japan, China relations strained over ownership of East China Sea islets, Japanese wartime aggression
China has revealed UNESCO has accepted its application to include documentation of the 1937 Nanjing Massacre and the issue of so-called "comfort women" -- sex slaves used by Japanese soldiers during World War II -- into a program that preserves important historical material relating to world heritage.
UNESCO says it has received China's application to list "comfort women" but that no decision will be made until 2015.
The issue threatens to put further strain on Sino-Japanese relations, which have deteriorated in recent years over conflicting territorial claims in the region as well as Japan's wartime past.
Earlier this year, Japan applied for a collection of diaries and wills of kamikaze pilots from World War II, to be registered with the same UNESCO body.
The organization's Memory of the World program has been collecting important world documents since 1992, preserving precious and potentially threatened material. Its archives include the diary of Ann Frank and the 13th century Magna Carta -- a document that challenged the English monarchy's absolute rule, making it subject to the rule of law.
Chinese state media, Xinhua news agency, reported Thursday that the latest UNESCO-registered documents are primary sources that point to atrocities committed by Japanese troops occupying the Chinese city of Nanjing from December 13, 1937 to March 1, 1938.
This includes historical files recording the recruitment of "comfort women" related to the Japanese Kwantung Army, police stationed at the Shanghai International Settlement, the Japanese-supported puppet regime of Wang Jingwei, and the Central Bank of Manchou, as well as written confessions by Japanese troops, according to a statement from the Ministry of Foreign Affairs (MoFA).
Japan forced about 200,000 women into sex slavery, luring them to "comfort stations" set up throughout East Asia by the Japanese military from 1932 until the end of the war, according to nonprofit advocacy group Washington Coalition for Comfort Women Issues.
CNN has widely reported on the stories of the women. Many have said they were kidnapped. For resisting sex, some say, they were raped and beaten. China insists that Japan has not officially apologized for these wartime atrocities.
The Nanjing Massacre refers to the invasion of the city of Nanjing in China's Jiangsu province in 1937, during which time Japanese troops killed an estimated 300,000 people in a rampage of rape, murder, and looting.
Japanese conservatives insist that the mass murders and rapes did not take place.
China submitted the wartime documents to the UNESCO program "to prevent the miserable and dark days from coming back again," said MoFA spokeswoman Hua Chunying.
Japan has condemned China's decision to register the wartime documents, calling it a political move during a sensitive period in the two countries' relations.
"It is extremely regrettable that China used UNESCO politically to unnecessarily play up the negative legacy on a certain period of the history of the both countries when we need effort for the improvement on the bilateral relationship," Chief Cabinet Secretary Yoshihide Suga said on Wednesday.
Suga added: "We cannot deny that there was murders and looting against non-combatants after the Japanese imperial army entered into Nanjing. However there are several estimates about the scale and it is difficult for the government to determine clearly."