U.S. Secretary of State John Kerry is trying to arrange direct talks between South Sudan's president and the country's top rebel leader, in hopes of ending four months of deadly violence.
Kerry spoke to reporters in Juba on Friday after meeting for more than an hour with President Salva Kiir.
"I told President Kiir that the choices that both he and the opposition face are stark and clear. And that the unspeakable human costs that we have seen over the course of the last months and which could even grow if they fail to sit down are unacceptable to the global community."
Later, Kerry spoke by phone to rebel chief Riek Machar. U.S. officials say Machar did not make a commitment to attend talks, but also did not rule it out.
Kerry said projected talks between the men could take place next week in Ethiopia's capital, where peace talks between South Sudanese government and rebel forces have made little progress.
Also Friday, the United Nation's special advisor on the prevention of genocide, Adama Dieng, said that the conflict in South Sudan has quickly deteriorated into ethnic violence and said there is a risk it could turn into genocide.
"The violence that broke out on 15 December following a power struggle within the rulings Sudan People's Liberation Movement (SPLM) between former Vice President Riek Machar and President Salva Kiir, has quickly degenerated into ethnic violence. The recent brutal attacks in Bentiu, Unity State and Bor, Jonglei state, seem to have changed the trajectory of the conflict. And if such attacks are not immediately halted could plunge the country into serious violence that could spiral out of control. In the current situation we see elements that we could categorize as risk factors of genocide and other atrocity crimes.''
Fighting erupted across South Sudan in December after Mr. Kiir accused Machar of attempting a coup. The violence has killed thousands and displaced more than 1 million people.
Kerry is on a multi-nation tour of Africa that has largely focused on security and human rights issues.
On Saturday, Kerry is due to meet in Addis Ababa with Somali President Hassan Sheikh Mohamud. Kerry and the Somali leader are expected to discuss the country's efforts to fight al-Qaida-linked militant group al-Shabab.
On Thursday, Kerry held talks with AU officials on the Central African Republic, where Muslims continue to flee their homes to escape attacks by Christian militia.
Later on Saturday, he travels to the Democratic Republic of Congo's capital to meet with President Joseph Kabila. The two will discuss recent security gains against rebel groups in volatile eastern Congo.
After a stop in Angola, Kerry returns to Washington on Monday.