"For five months, fighting in South Sudan has robbed that country of hope and denied its people the peace and prosperity they deserve," National Security Adviser Susan Rice said. (Agencies)
"The agreement South Sudan President Salva Kiir and Riek Machar signed today in Addis Ababa, Ethiopia, holds the promise of bringing the crisis to an end," she said.
President Salva Kiir and rebel boss Riek Machar inked a deal "ending the war", under international pressure to stem bloodshed and avert famine and genocide.
The two rivals agreed to end all fighting within 24 hours of signing the document.
US Secretary of State John Kerry, in a separate statement, said the 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.
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," Kerry said.
Noting that this agreement presents an opportunity to start a path towards peace that must not be lost, Kerry said: "We will do all we can to help."
In their statements, Kerry and Rice acknowledge the effort of the Chairman of the Intergovernmental Authority for Development, Prime Minister Hailemariam Desalegn, to convene the meeting and to mediate the positive outcome between the two rival leaders.
"The people of South Sudan have suffered too much for far too long. In this most recent crisis alone, over one million people have been displaced, even more now face the prospect of famine, and, as the recent UN Mission Human Rights report found, there have been human rights abuses on a massive scale committed by both sides," Kerry said.
"We urge President Kiir and Mr Machar to move swiftly to honour the agreement in word and deed by ending the violence and negotiating in good faith to reach a political agreement that can ensure stability, prosperity and peace for all of South Sudan's people," Rice said.
The war has claimed possibly tens of thousands of lives and forced over 1.2 million people to flee their homes. Aid agencies warn that South Sudan is now on the brink of Africa's worst famine since the 1980s.
"For five months, fighting in South Sudan has robbed that country of hope and denied its people the peace and prosperity they deserve," National Security Adviser Susan Rice said.