The United Nations became more involved with the conflict in 1976. In 1979, the Organisation of African Unity (OAU) also became involved in the Settlement Plan. The UN and the OAU introduced a plan in the late 1980s and negotiated with parties for about five years. On July 20, King Hassan of Morocco agreed to a referendum "for the self-determination" of the Western Saharan people. Once eligible voters were identified for the referendum, the people of Western Sahara would vote on whether they would be independent or become part of Morocco. The plan "was accepted in principle by Morocco and Polisario at the end of August 1988." The referendum was written up in Security Council Resolution 658. The United Nations Security Council became involved in 1991. The agreement resulted in a cease-fire between Morocco and Polisario, which went into effect on September 6, 1991. The cease-fire was treated as a separate issue from the rest of the Settlement Plan, which included voter registration for the referendum, by Pérez de Cuéllar, the current United Nations Secretary General. The referendum was written up as Security Council Resolution 690 on April 29, 1991. Resolution 690 also created the United Nations Mission for the Referendum in Western Sahara (MINURSO).