China has released previously confidential Japanese wartime documents, including some about “comfort women” forced to serve in military brothels during World War II, state media reported.

The publication comes during a fraught period in Japan-China relations. Last week, 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 Prime Minister Shinzo Abe to water down a 1993 apology to the comfort women, as they are euphemistically called in Japan. These politicians have said there is no evidence of large-scale coercion by the government or military.

After testing the waters for a revision, Abe backed down last month and said Tokyo will not revise the apology, known as the Kono statement.

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 reports about the massacre were exaggerated for propaganda purposes. Many of Japan’s wartime records were destroyed by Japan.

The release coincided with the publication Saturday of over 110,000 previously confidential Japanese government and military documents from war times 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 are “still fresh in our memory.” His comments prompted an angry response from Tokyo.

In December, Abe provoked China’s ire when he visited Yasukuni Shrine in Tokyo, where both war dead and Class-A war criminals are honored. Last week, more than 150 lawmakers and two members of Abe’s Cabinet visited Yasukuni.

Territorial disputes also dog the relationship. These are centered on the Senkaku Islands, a string of uninhabited rocks in the East China Sea that the Chinese call Diaoyu and Taiwan calls Tiaoyutai.

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

Last week, Tokyo Gov. 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.

This followed a trip to Japan in early April by Hu Deping, son of late reformist Chinese leader Hu Yaobang. Hu met with several senior statesmen and held a confidential meeting with Abe.