National Security Adviser Susan Rice welcomed the peace agreement in a statement, saying it "holds the promise of bringing the crisis to an end."
A cease-fire in January between South Sudan President Salva Kiir and rebel leader Riek Machar fell apart within days.
Rice urged that Kiir and Machar follow up on the new peace deal signed in Addis Ababa, Ethiopia, by "ending the violence and negotiating in good faith to reach a political agreement."
Ethnically targeted violence in the world's youngest country broke out in December, killing thousands of people and forcing more than 1.3 million to flee their homes. The U.N. Security Council has expressed "horror" at recent killings of civilians.
Friday's meeting in Addis Ababa was the first face-to-face encounter between Kiir and Machar since the mass violence began, and it came a week after U.S. Secretary of State John Kerry met with Kiir to urge a revitalization of peace talks.
U.N. Secretary-General Ban Ki-moon said during a visit to South Sudan this week that the country has seen serious human rights violations. A new U.N. report said gross violations of human rights and international humanitarian law have been committed.
Much of the violence has been ethnic in nature and carried out by troops loyal to Kiir, an ethnic Dinka, and rebels loyal to former Vice President Machar, an ethnic Nuer.
International pressure had been growing for at least a brief cease-fire to allow residents to plant their fields, with the U.N. and aid groups warning that if crops aren't planted this month, the country could face mass hunger or famine. Tens of thousands of civilians already have been taking refuge in U.N. compounds across the country for months.
The Security Council in recent days discussed sanctions, an arms embargo and a referral of the South Sudan situation to the International Criminal Court as ways to apply pressure on the warring sides.
South Sudan is a largely Christian nation that broke off from the Muslim-dominated Sudan after a 2011 referendum. The fighting is an embarrassment to the U.S., which has provided hundreds of millions of dollars in aid and has been its strongest international champion.
