Tokyo is considering suspending or reducing its financial contribution to UNESCO in response to the U.N. body’s acceptance, through closed-door processes, of Beijing’s documents on the 1937 Nanking Massacre, Chief Cabinet Secretary Yoshihide Suga said Tuesday.

On Friday, UNESCO announced that it had registered what China claims are historical documents on the Nanking Massacre, also known as the Rape of Nanking, in its Memory of the World program.

Suga claimed the process of registration happened without verification by third-party experts, lacked “transparency” and violated the political “neutrality” of UNESCO.

“The (screening process) takes place amid concealment and secrecy,” Suga told a news conference Tuesday morning. “The Japanese government has not even been allowed to see the documents.

“We will consider all possible options, including suspension of payment,” he added.

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If Japan does suspend payment, it will affect UNESCO’s operations to a considerable degree.

In fiscal 2014, Japan’s contribution to UNESCO totaled ¥3.7 billion, making it the No. 1 donor as the U.S. suspended payments to the body the same year.

Later in the day, a high-ranking Japanese official said Tokyo, by suggesting a review of its financial contribution, will urge UNESCO to reform the screening system for the Memory of the World program so as to enhance its transparency.

During the same news conference, Suga said Japan and China have conflicting opinions about what has been confirmed by historical evidence with regard to the incident.

Nonetheless, UNESCO “one-sidedly” accepted the documents submitted by China, Suga claimed.

“An international organization should be fair and neutral,” he said. “We’d like to strongly seek reform of this system, which can be used for political purposes.”

The “Documents of Nanjing Massacre” include court papers from the International Military Tribunal for the Far East, documents from Chinese military tribunals and reports allegedly written by Japanese military police.

They also include photos of the killings said to have been taken by the Imperial Japanese Army, as well as documentary film shot by American priest John Magee.

China has claimed about 300,000 Chinese were slaughtered by Japanese soldiers and numerous women were raped after Japan occupied Nanking in December 1937. The city is currently known as Nanjing.

Japan officially has admitted that Japanese troops killed noncombatants and that plundering took place in Nanking.

But it also has claimed that the number of victims remains undetermined and protested Beijing’s allegations that 300,000 were massacred.

During the news conference, Suga did say “it cannot be denied” that Japanese troops killed Chinese noncombatants and looted the city. He did not elaborate further.

China also sought registration of what it claims to be historical documents on “comfort women,” or those forced to provide sex at Japanese wartime military brothels, for UNESCO’s memory program. However, that application was not accepted.

Prime Minister Shinzo Abe has often been regarded as a historical revisionist, as he has suggested he wants to change some of Japan’s key official war apology statements.

Facing criticism, Abe eventually pledged this year to uphold all such apologies, including those regarding “comfort women.”

In March 2014, Japan strongly protested against The New York Times when the paper claimed in its editorial that Abe and other Japanese nationalists “still claim that the Nanking massacre by Japanese troops in 1937 never happened.”

Tokyo claimed that Abe “has never made such remarks” to deny the Nanking Massacre itself.

Meanwhile, some right-wing nationalist lawmakers, including those from Abe’s own ruling Liberal Democratic Party, have indeed claimed that the Nanking Massacre never happened.

Chinese media outlets welcomed UNESCO’s registration of the documents.