KHARTOUM, Sudan — South Sudanese rebels refused on Tuesday to sign the latest draft of a peace deal that would have ended a devastating civil war, a new setback in the five-year conflict.

President Salva Kiir and the leader of the main rebel group, Riek Machar, signed a cease-fire and power-sharing agreement earlier this month, having reached a preliminary deal in June that intended to end the fighting that first broke out in 2013.

But Mr. Machar and another rebel group refused on Tuesday to sign the latest draft of the agreement, saying that disputes over power sharing and a new constitution had not been sufficiently addressed.

“This is an unfortunate development that will not reflect well on the impartiality of the mediators and will throw doubt onto the whole process,” the rebels said in a statement.