By Alexandra Harney

SHANGHAI (Reuters) - China has released previously confidential Japanese wartime documents, including some about comfort women forced to serve in military brothels during World War Two, state media reported.

The publication comes during a fraught period in Japan-China relations. Last week, Japan's Mitsui O.S.K. Lines Ltd paid about $29 million for the release of a ship seized by China over a dispute that dates back to the 1930s war between the two countries.

The 89 documents released from archives in northern Jilin province include letters written by Japanese soldiers, newspaper articles, and military files unearthed in the early 1950s, state media said. Why they had not been released until now was not immediately clear.

Nationalist politicians in Japan have been urging Japanese prime minister Shinzo Abe to water down a 1993 apology to comfort women. These politicians have said there is no evidence of large scale coercion by government authorities or the military.

Abe said last month that Tokyo would not revise this apology.

The Jilin documents include Japanese records on the exploitation of "comfort women" by troops as well as details of the Nanjing Massacre that began in December 1937.

China and Japan disagree on the number of people killed in the massacre. Some nationalist Japanese politicians have argued that the reports about the massacre have been exaggerated for propaganda purposes. Many of Japan's wartime records were destroyed.

The documents' release coincided with the publication on Saturday of more than 110,000 previously confidential Japanese government and military documents from times of war by China's Thread Binding Books Publishing House.

History is a live issue between Japan and China. In a speech in Berlin last month, Chinese President Xi Jinping noted that the atrocities in Nanjing were "still fresh in our memory". His comments prompted an angry response from the Japanese government.

Last December, Abe provoked China's ire when he visited Tokyo's Yasukuni Shrine, where both war dead and war criminals are honored. Last week, more than 150 Japanese lawmakers and a member of Abe's cabinet paid their respects at Yasukuni.

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Territorial disputes also dog the relationship. These are centered on a string of uninhabited islands in the East China Sea the Chinese call the Diaoyu islands and Japanese call the Senkaku islands.

However, there are signs of warmth amidst the chill as well.

Last week, Tokyo governor Yoichi Masuzoe visited Beijing, meeting with China's Vice Premier Wang Yang and former top Chinese diplomat Tang Jiaxuan and passing on a message from Abe that he hoped bilateral ties would improve.

That visit followed a trip to Japan in early April by Hu Deping, the son of late reformist Chinese leader Hu Yaobang. Hu met with several senior statesmen. His visit included a confidential meeting with prime minister Abe.

(Editing by Kim Coghill)