BEIJING, Feb. 9 -- After a director of Japanese public broadcaster NHK attempted to deny the Nanjing Massacre earlier this week, historians have said that the facts are undeniable.

Naoki Hyakuta, one of NHK's governors, said in a speech that the Nanjing massacre never happened, just a few days after the NHK's new chief comments about "comfort women", which sparked widespread anger.

Ben Dorfman, an associated professor who is studying cross-cultural history at Aalborg University in Denmark, said in an email interview with Xinhua on Saturday that "the Rape of Nanjing" was written in world history books.

He mentioned two books -- "A History of the World from the 20th to the 21st Century" published by Routledge and "History of World Societies" by Houghton Mifflin Company, which are required reading for freshmen at the university.

History books record Japanese troops' brutal behavior, including rape and looting, after Nanjing fell in December 1937, with black and white photographs.

"History is history and cannot be changed," said Zhu Chengshan, curator of the Nanjing Massacre Memorial Hall.

The denial of the massacre shows Japan's intention to throw off its image as the aggressor in WWII, not historical ignorance, Zhu went on.

Documents show that Japan occupied Nanjing on Dec. 13, 1937 and in the following six weeks, the army committed arson and looted the city, killing more than 300,000 civilians and unarmed soldiers.

Japan's right wing have denied the massacre repeatedly for political purposes, said Zhang Xianwen, director of the research institute for the Nanjing Massacre at Nanjing University.

Bjorn Hess, a postgraduate at Aalborg University, is incredulous that anyone would deny the massacre, which he learned about in high school.

But Dorfman and Hess said they did not know much about the details, especially the number of victims.

The international community has limited access to details of the events, and misleading media reports create confusion, Zhang said.

Sun Zhaiwei of the Jiangsu Provincial Academy of Social Sciences said 300,000 victims were confirmed by the International Military Tribunal for the Far East and the Nanjing War Crimes Tribunal and the number is not subject to any doubt.

According to the tribunals, Japanese invaders killed more than 200,000 civilians and soldiers in Nanjing, excluding those whose remains were cremated or dumped in the Yangtze. At least 300,000 Chinese civilians and soldiers were shot or buried alive.

The judgment of both tribunals are legal documents, recognized by Japan.

China investigated the massacre after WWII, despite difficulty in collecting first-hand evidence in post-war conditions, Zhang Xianwen said.Only about 200 survivors remain alive today, and China has a duty to disseminate the facts of the massacre through publications, documentaries and movies like "City of Life and Death," (sometimes called "Nanjing! Nanjing!") and "The Flowers of War".

International seminars and exhibitions could help spread the truth of the Nanjing Massacre. The memorial, built in 1985, has hosted several overseas exhibitions in the U.S., Italy, Russia, the Philippines and so on.

"Nanjing Massacre is undeniable. We should never allow the historical tragedy to be repeated," Zhu Chengshan said.