In releasing the documents realted to Unit 731, the US points out the UK also has documents related to this.
"The compartmentalization of the war in Asia also diminished the possibility that one
nation would end up with the lionís share of Japanese documentation. Unlike the German
case, there was no one central repository for Asia-specific war crimes documentation.
British Empire forces, for example, took charge of Japanese materials in Southeast Asia.
Returning colonial authorities in Indochina and the Dutch East Indies gathered material
for their war crimes trials. As many as 40,000 U.S. Marines garrisoned transportation
centers in north China from October 1945 into 1947 and accepted the surrender of
Japanese units, but otherwise there was little U.S. presence in the huge country, and U.S.
units collected relatively few Japanese documents from China."