In early 1991, non-Arabs of the Zaghawa tribe of Sudan attested that they were victims of an intensifying Arab apartheid campaign, segregating Arabs and non-Arabs (specifically people of sub-Saharan African descent). Sudanese Arabs, who controlled the government, were widely referred to as practicing apartheid against Sudan's non-Arab citizens. The government was accused of "deftly manipulat(ing) Arab solidarity" to carry out policies of apartheid and ethnic cleansing.
When did the Arab apartheid intensify? 
early 1991