US Secretary of State John Kerry on Saturday put fresh pressure on South Sudan's president and rebel leader to hold face-to-face peace talks, urging them to pull the world's youngest nation "back from the abyss".
In a policy speech in the Ethiopian capital and headquarters of the African Union, Kerry also vowed to remain personally engaged in efforts to end South Sudan's four-month-old civil war.
"Yesterday I was in South Sudan. I saw how a new nation and once hopeful vision for the future can be challenged by old grudges degenerating into violence," he said of the rivalry between President Salva Kiir and former vice president turned rebel leader Riek Machar.
"I expressed my grave concerns to President Kiir about the deliberate killings of civilians on both sides of the conflict and he agreed to embark on negotiations to form a transitional government that can lead this new nation back from the abyss. I also called the former vice president Riek Machar and urged him to do the same," Kerry added.
"If both sides do not take bold steps to end the violence, they risk plunging South Sudan into greater desperation and even famine. They will completely destroy what they claim they are fighting for," he said.
"In the days to come, I will continue my personal engagement with both sides."
Officials said Kerry has brandished the threat of targeted sanctions against Kiir and Machar, and hopes are high that the two leaders will meet in the Ethiopian capital in the coming days -- for the first time since the fighting began on December 15.
After their talks in South Sudan's capital Juba on Friday, Kerry said President Kiir had expressed his willingness to meet with Machar under Ethiopian mediation.
Kerry spoke by telephone late Friday with Machar -- who has always said he was open to talks to end the conflict -- but he has yet to officially confirm attendance.
The United States has also been under pressure to intervene, having been a key backer of South Sudan's push for independence from Khartoum and having poured in billions of dollars in aid to the country since it split from Sudan in 2011.
The conflict started with Kiir accusing Machar of attempting a coup. Machar then fled to the bush to launch a rebellion, insisting the president had attempted to carry out a bloody purge of his rivals.
The violence has left thousands dead -- and possibly tens of thousands -- with at least 1.2 million people forced to flee their homes, many living in appalling conditions in overstretched UN bases.
Aid agencies are also warning that South Sudan is on the brink of Africa's worst famine since the 1980s, with the United Nations demanding at least a one-month-long truce so that crops can be planted and food stocks boosted.