TOKYO - Japan's government said Tuesday that it is considering cutting funding for UNESCO after the United Nations world heritage body included documents on the Nanjing Massacre in its Memory of the World register, a move that Tokyo criticized as biased.

Chief Cabinet Secretary Yoshihide Suga, the government's chief spokesman, told reporters that UNESCO's decision reflected only China's views on the 1937 assault on the Chinese city, when Japanese troops massacred tens of thousands of civilians in what has become known as the Nanjing Massacre.

Chinese and Japanese estimates vary regarding how many people died in the massacre, viewed as one of the worst atrocities of the Second World War era. Estimates on the number of deaths range from 40,000 to 300,000.

"There is a big discrepancy of views between Japan and China, and the decision reflecting a unilateral view turns the issue into a political problem," Suga said. "We are considering all measures (of protest), including suspension of our funding contributions" to UNESCO.

"The decision making process lacked transparency," he said. "We were not even allowed access to the contents of the Chinese documents."

According to the Japanese Foreign Ministry, Japan contributed 3.72 billion yen (about $31 million) to UNESCO in 2014, or 10.8 per cent of its budget assessed for the UN

UNESCO's budget for the two-year 2014-2015 period is $653 million. Japan has also set up various trust funds to support work on world and cultural heritage efforts.

Suga said Japan would also seek reform of UNESCO, which was the first UN organization Japan joined after the Second World War, in its 1951 return to the international community.

While Japan objected to the inclusion of the documents on the Nanjing Massacre, it succeeded in having some of its own candidates for the memory list included, including details on detainees held in Siberia after the Second World War.

Materials submitted by China for inclusion in the Memory of the World Register in 2014 included documents about the period of the massacre, about the postwar investigation and trials of war criminals documented by the Chinese National Government's Military Tribunal in 1945-47 and 1952-56 files from China's judiciary.

UNESCO did not agree to register documents submitted by China about women forced to provide sex to Japanese Imperial Army troops during the war.