Sudan's state-run news agency says Darfur residents have begun voting on whether to unify the region's five states, in a referendum the U.S. said would "undermine the peace process."

The Khartoum government divided the region into three states in 1994, diluting the power of the Fur tribe, the region's largest. Darfur's division and the marginalization of the Fur were among the main factors behind the fighting that erupted in 2003, which led to the deaths of 300,000 people and allegations of crimes against humanity.

The government divided Darfur into five states in 2012.

The U.S. State Department has said the three-day referendum that began Monday is unlikely to be fair, saying "insecurity in Darfur and inadequate registration of Darfuris residing in internally displaced persons (IDPs) camps prohibit sufficient participation."