Nazi Germany used six extermination camps, also called death camps, or killing centers, in Central Europe, primarily in German-occupied Poland, during World War II to systematically murder over 2.7 million people—mainly Jews—in the Holocaust. The victims of death camps were primarily murdered by gassing, either in permanent installations constructed for this specific purpose, or by means of gas vans. The six extermination camps were Chełmno, Bełżec, Sobibor, Treblinka, Majdanek and Auschwitz-Birkenau. Extermination through labour was also used at the Auschwitz and Majdanek death camps. Millions were also murdered in concentration camps, in the Aktion T4, or directly on site. Additionally, camps operated by Nazi allies have also been described as extermination or death camps, most notably the Jasenovac concentration camp in the Independent State of Croatia.