South Sudan's president and rebel chief signed a ceasefire deal Friday vowing to end nearly five months of civil war, with the US urging both sides to "swiftly" implement their promises.
President Salva Kiir and rebel boss Riek Machar, who shook hands and then prayed together, "agreed that immediately all hostile activities will stop within 24 hours from the signing of this agreement", said head mediator Seyoum Mesfin, from the East African regional bloc IGAD.
"Fighting will stop," he added.
Kiir, explaining his olive branch to his bitter rival, told reporters that as leader he had in the past accepted compromises and had "been in a position to make peace with everybody".
Machar, who swapped his military fatigues for a business suit, said he was "happy" at the signing of the agreement.
The peace deal, which followed intense lobbying from world leaders with Washington slapping sanctions on senior military commanders, came following UN warnings that crimes against humanity had likely been carried out in the still raging conflict.
"Today's agreement to immediately stop the fighting in South Sudan and to negotiate a transitional government could mark a breakthrough for the future of South Sudan," US Secretary of State John Kerry said in a statement.
"The hard journey on a long road begins now and the work must continue," he added.
"We urge both leaders to take immediate action now to ensure that this agreement is implemented in full and that armed groups on both sides adhere to its terms."
The deal recommits to an earlier ceasefire, in tatters ever since it was signed in January.
The rivals "agreed that a transition government offers the best chance to the people of South Sudan" with the promise of fresh elections, without giving a date, Seyoum said.
Both sides also "agreed to open humanitarian corridors... and to cooperate with the UN" to ensure aid is delivered to the more than five million people in need, he added.
But while both leaders promised peace, fierce fighting still rages, amid United Nations warnings of the risk of severe famine and genocide.
The war has claimed thousands -- and possibly tens of thousands -- of lives, with more than 1.2 million people forced to flee their homes.
Aid agencies are warning that South Sudan is now on the brink of Africa's worst famine since the 1980s.
Top African Union official Smail Chergui, the pan-African bloc's peace and security commissioner, said that while the agreement was welcomed, "even with the signing, given the current crisis, the restoration of peace in South Sudan will not be easy".
UN rights chief Navi Pillay said the two leaders must "stop the killing, before the fire they have ignited makes the entire country go down in flames".
Pillay, a former head of the UN genocide court for Rwanda, said she recognised in the UN report "many of the precursors of genocide".
These included hate radio urging rape and "attacks on civilians in hospitals, churches and mosques, even attacks on people sheltering in UN compounds -- all on the basis of the victims' ethnicity".
The conflict, which started as a personal rivalry between Kiir and Machar, has seen the army divide along ethnic lines, pitting members of Kiir's Dinka tribe against Machar's Nuer.
Testimonies in a report this week by Amnesty International describe civilians including children executed by the side of the road "like sheep" and other victims "grotesquely mutilated" with their lips sliced off.
The conflict erupted on 15 December with Kiir accusing Machar of attempting a coup. Machar then fled to the bush to launch a rebellion, insisting that the president had attempted to carry out a bloody purge of his rivals.