UNESCO, a top United Nations agency, has added Nanjing Massacre historical documents to its Memory of the World Register, a move welcomed by Beijing but slammed by Tokyo, CNN News reports.

Japanese Chief Cabinet Secretary, Yoshihide Suga said it was considering protest measures, including suspending its funding contribution to the UN body.

Suga also complained that the UNESCO selection process was not transparent.

"The decision-making process for the Memory of the World is extremely obscure. We were not even allowed access to the contents of the document," Suga said.

The documents include 11 sets of files on the Nanjing Massacre, also known as the Rape of Nanking, ranging from film, photographs and text recorded between 1937 and 1948, Chinese state media reports.

"China will ensure these valuable documents are protected and circulated, and make them play a positive role in remembering history, cherishing peace, looking into the future and safeguarding human dignity," Chinese Foreign Ministry spokesperson Hua Chunying told Chinese state media outlet Xinhua.

China filed the application with UNESCO more than a year ago.

The UNESCO Memory of the World Register was established in 1992 with the aim of preserving documentary heritage. It includes everything from the archives of the Dutch East India company and the diary of Anne Frank to the music of Romantic composer Johannes Brahms.