South Sudan's president Salva Kiir and rebel commander Riek Machar are both committed to holding direct talks to resolve the four-month conflict that has killed thousands, John Kerry, the US secretary of state, said.
Kerry, currently on a tour of several African countries, pressed the warring leaders to set a date for face-to-face talks, urging the UN Security Council to consider sanctions to stop attacks on civilians.
"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," Kerry said on Saturday of the rivalry between the two men as he continued his African tour in Addis Ababa, Ethiopia's capital.
"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. In the days to come, I will continue my personal engagement with both sides."
The meeting, which would be the first face-to-face talks since the conflict erupted in December, is to be held in Ethiopia where both sides have previously held talks without success.
"Salva Kiir has said he will meet directly with former vice president Machar," Samantha Power, the US ambassador to the UN, told the Security Council on Friday, according to the AFP news agency.
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"We heard many promises from South Sudanese leaders before without follow-up ... We hope for the sake of the people of South Sudan that this time it is different," she added.
"We urge president Kiir and former vice president Machar to swiftly agree on a date for face-to-face talks."
The US, a key backer of South Sudan's independence in 2011, is under pressure to intervene having given the country billions of dollars in aid.
With violence rising between the ethnic Dinka and Nuer, members of the international community have drawn parallels between the situation in South Sudan and the build-up to events that led to genocide in Rwanda 20 years ago.
Kiir is from the Dinka ethnic group, the country's largest, while Machar belongs to the Nuer but prominent South Sudanese from the Dinka ethnic group are backing Machar in the conflict.
Last month Barack Obama, the US president, authorised punitive sanctions, including the seizure of assets and visa bans, against anyone in South Sudan deemed to be threatening peace efforts.
Power said the UN should also consider "urgently whether to put in place parallel targeted sanctions so as to try to deter outrageous attacks on civilians".
She said the United States would work towards revising the mandate of the UN mission in South Sudan to put greater emphasis on the protection of civilians, rights and the delivery of aid.
"The culture of impunity must end," said Power.