KHARTOUM, Sudan — Rival military groups fighting in Sudan have agreed to extend their cease-fire by another 72 hours, brokers U.S. and Saudi Arabia confirmed, while an ethnic conflict flared up in the state of West Darfur. Violence has broken out between the Masalit of African origin and groups of Arab origin in the western Sudanese state, according to reports. On Wednesday, U.N. Special Envoy…