Washington - President Barack Obama urged South Sudan's government and rebels on Thursday to implement a new ceasefire as soon as possible and cement it with an inclusive political dialogue.
The deal committed both sides to halt fighting within 24 hours and end five weeks of conflict that has left thousands dead.
"I welcome today's signing of a cessation of hostilities agreement in South Sudan, which is a critical first step toward building a lasting peace," Obama said in a statement.
"Now, South Sudan's leaders need to work to fully and immediately implement the agreement and start an inclusive political dialogue to resolve the underlying causes of the conflict.
"The full participation of political detainees currently being held by the government of South Sudan will be critical to those discussions, and we will continue to work to expedite their release."
Obama noted that the United States had been a strong supporter of the fledgling nation of South Sudan, but warned that its leaders now needed to win back the trust of their people.
The agreement was signed in the Ethiopian capital Addis Ababa by representatives of South Sudan's President Salva Kiir and rebel delegates loyal to ousted vice president Riek Machar.
Mediators from the East African regional bloc IGAD, which brokered the peace talks, said the deal will put in place a verification and monitoring mechanism for the truce.
The US special envoy to South Sudan and Sudan, Donald Booth, earlier this month met Machar along with regional mediators.
After initial clashes broke out in the capital Juba more than a month ago, the conflict rapidly escalated into all-out war between the regular army, backed by Ugandan troops, and breakaway army units and other militia.
The violence also took on an ethnic dimension as members of Kiir's Dinka tribe clashed with Machar's Nuer group.
Aid workers and analysts say up to 10 000 people have died, while close to half a million have been forced to flee their homes, with atrocities allegedly committed by both sides.
Jan Egeland, a former UN aid chief and now head of the Norwegian Refugee Council, told AFP the scale of atrocities and war crimes was as bad as that seen in Syria or Somalia.
Responding to those comments, White House spokesperson Jay Carney said "those who have committed atrocities must be held accountable".
The state department said it was important to ensure help can reach the hundreds of thousands of people who have been affected by this conflict.
"To this end, we call on all parties to facilitate the immediate and unfettered provision of humanitarian assistance to all those in need in South Sudan, regardless of where they are located," said state department deputy spokesperson Marie Harf.