Kiir said he would release political prisoners, but with no sign of a political deal with rebels. It was not clear whether the ceasefire would take hold.
U.N. and other countries welcomed the announcements, but warned they would be closely scrutinized.
South Sudan has been mired in a civil war since 2013, when Kiir fired his deputy.
The conflict, fanned by ethnic rivalries, has sparked Africa’s worst refugee crisis since the 1994 Rwandan genocide and plunged parts of the country into famine.