South Sudanese soldiers and opposition rebel fighters raped hundreds of women and girls during fighting in July, the United Nations said on Thursday, as the world’s youngest nation teetered on the brink of political collapse and another round of civil war.

The U.N. High Commissioner for Human Rights Zeid Ra’ad Al Hussein said preliminary investigations into the hostilities between the army loyal to President Salva Kiir, an ethnic Dinka, and rebels loyal to ousted vice president and opposition leader Riek Machar, an ethnic...