U.S. Secretary of State John Kerry is trying to arrange direct talks between South Sudan's president and the 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.
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.
Kerry is now back in Addis Ababa, where he is due to meet Saturday 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.
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. The United States has threatened sanctions against those responsible for the violence.
Kerry is on a multi-nation tour of Africa that has largely focused on security and human rights issues.
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.