In 1918, the predominantly Armenian-populated region of Nagorno-Karabakh was claimed by both the Azerbaijan Democratic Republic and the First Republic of Armenia when both countries became independent after the fall of the Russian Empire, and a brief war over the region broke out in 1920. The dispute was largely shelved after the Soviet Union established control over the area, and created the Nagorno-Karabakh Autonomous Oblast (NKAO) within the Azerbaijani SSR in 1923. During the fall of the Soviet Union, the region re-emerged as a source of dispute between Armenia and Azerbaijan. In 1991, a referendum held in the NKAO and the neighbouring Shahumian region resulted in a declaration of independence. The ethnic conflict led to the 1991–1994 Nagorno-Karabakh War, which ended with a ceasefire along roughly the current borders. According to UNHCR, the conflict resulted in over 600,000 internally displaced persons within Azerbaijan.