Secretary of State
Kerry said Kiir was willing to fly to Ethiopia and meet with former South Sudan Vice President Riek Machar, a session Kerry said would be "critical" to resolve the crisis.
Kerry was due to speak to rebel leader Machar by phone later Friday to ask him to meet with Kiir in the Ethiopian capital.
A political struggle between Kiir and Machar late last year plunged the country into ethnic violence and derailed international promises of loans and aid to develop the nation.
A meeting between Kiir and Machar would be the first since the fighting broke out. Tensions rose late last year after Machar and others in the ruling party announced they would contest presidential elections due in 2015. They accuse Kiir of increasing autocracy after he sacked his entire government in July.
Kerry told reporters Kiir had agreed to talks on a transitional government - a common formula to end civil wars in Africa. The arrangement often entails all parties joining in a power-sharing agreement, leading up to elections. But while transitional governments stop the fighting, they often fail to address the underlying conflicts, which may re-emerge later, triggering new fighting.
Kerry said Kiir agreed to "take forceful steps in order to begin to move to end the violence and implement the cessation of hostilities agreement and to begin to engage on a discussion with respect to a transition government." Kerry offered no details on what form a transitional South Sudan government would take.
He said he told Kiir that both he and Machar faced a stark choice.
Both sides signed a cessation of hostilities agreement in January, but neither has observed it. Before flying into Juba, Kerry raised the threat of U.S. sanctions on those encouraging violence in South Sudan. U.N. Human Rights Chief Navi Pillay this week met with Kiir and Machar and later blasted both for their failure to stop the fighting.
Talks have been continuing, on and off, between the two sides for months, with no progress. Fighting and back-and-forth ethnic massacres have persisted.
The conflict has set back South Sudan's economy and development and dashed hopes that investors, who in December came to Juba in their hundreds for a highly successful investment conference, will materialize.