200,000 ethnic Serbs, along with gypsies and Jews, were persecuted and murdered by the Axis-aligned Croatian Ustaše in Yugoslavia. Concurrently, Muslims and Croats were persecuted and killed by Serb nationalist Chetniks, with an estimated 50,000-68,000 victims (of which 41,000 were civilians). Also, more than 100,000 Poles were massacred by the Ukrainian Insurgent Army in the Volhynia massacres, between 1943 and 1945. At the same time, about 10,000–15,000 Ukrainians were killed by the Polish Home Army and other Polish units, in reprisal attacks. In Asia and the Pacific, the number of people killed by Japanese troops remains contested. According to R.J. Rummel, the Japanese killed between 3million and more than 10 million people, with the most probable case of almost 6,000,000 people. According to the British historian M. R. D. Foot, civilian deaths are between 10 million and 20 million, whereas Chinese military casualties (killed and wounded) are estimated to be over five million. Other estimates say that up to 30 million people, most of them civilians, were killed. The most infamous Japanese atrocity was the Nanking Massacre, in which fifty to three hundred thousand Chinese civilians were raped and murdered. Mitsuyoshi Himeta reported that 2.7 million casualties occurred during the Sankō Sakusen. General Yasuji Okamura implemented the policy in Heipei and Shantung. Axis forces employed biological and chemical weapons. The Imperial Japanese Army used a variety of such weapons during its invasion and occupation of China (see Unit 731) and in early conflicts against the Soviets. Both the Germans and the Japanese tested such weapons against civilians, and sometimes on prisoners of war. The Soviet Union was responsible for the Katyn massacre of 22,000 Polish officers, and the imprisonment or execution of thousands of political prisoners by the NKVD, along with mass civilian deportations to Siberia, in the Baltic states and eastern Poland annexed by the Red Army. The mass bombing of cities in Europe and Asia has often been called a war crime, although no positive or specific customary international humanitarian law with respect to aerial warfare existed before or during World War II. The USAAF bombed a total of 67 Japanese cities, killing 393,000 civilians, including from the atomic bombings of Hiroshima and Nagasaki, and destroying 65% of built-up areas. Genocide, concentration camps, and slave labour Nazi Germany, under the dictatorship of Adolf Hitler, was responsible for the Holocaust (which killed approximately 6million Jews) as well as for