The move could lay groundwork for a government to end bloodshed in the world's newest state.
Mr Kerry met Salva Kiir in the capital and emerged to announce a tentative agreement for talks in Ethiopia.
Mr Kerry said Mr Kiir's rival, former vice president-turned-rebel Riek Machar, is likely to join fresh talks.
The American said: "The unspeakable human costs we are seeing over the course of the last months, and which could even grow if they fail to sit down, are unacceptable to the global community.
"Before the promise of South Sudan's future is soaked in more blood, president Kiir and the opposition must work immediately for cessation of hostilities."
Thousands have died in fighting that began when Mr Kiir, a Dinka, accused Mr Machar, a Nuer, of plotting a coup last December.
An earlier ceasefire agreement reached last January was abandoned within days.