Documents related to the Nanjing massacre, a Japanese military rampage that Beijing says killed 300,000 people in 1937, were inscribed Friday in the Memory of the World register kept by the UN's scientific and cultural body.
The decision came after a two-year process during a meeting of UNESCO experts tasked with studying nominations from 40 countries.
A total of 47 new inscriptions were agreed at the meeting which ran from Sunday to Tuesday and was held in the United Arab Emirates.
The massacre known as the "Rape of Nanjing" is an exceptionally sensitive issue in the often-tense relations between Japan and China, with Beijing charging that Tokyo has failed to atone for the atrocity.
The Japanese military invaded China in the 1930s and the two countries fought a full-scale war from 1937 to 1945 that ended with Japan's defeat in World War II.
China says 300,000 people died in a six-week spree of killing, rape and destruction after the Japanese military entered Nanjing, although some respected foreign academics put the number lower.
China historian Jonathan Spence, for example, estimates that 42,000 soldiers and citizens were killed and 20,000 women raped, many of whom later died.
In Japan, however, some question that view. In February, a senior executive at Japan's publicly-funded TV broadcaster NHK denied the massacre, reportedly dismissing accounts of it as "propaganda".
Japan's official position is that "the killing of a large number of noncombatants, looting and other acts occurred", though adds "it is difficult to determine" the correct number of victims.
In April this year, Japan rebuffed protests about newly-approved textbooks after complaints that they failed to use the word "massacre" when referring to the mass slaughter of Chinese civilians in Nanjing, preferring the term "incident".
The Memory of the World register, set up in 1992, is aimed at preserving humanity's documentary heritage, and currently holds 348 documents and archives that come from countries all over the world.
"It is my deep and firm conviction that the Memory of the World Programme should be guided in its work to preserve documentary heritage and memory for the benefit of present and future generations in the spirit of international cooperation and mutual understanding, building peace in the minds of women and men," UNESCO director Irina Bokova said.