On Monday, President Salva Kiir initialed part of the power-sharing deal, but said he would wait until early September before finalising it.
Washington criticised Kiir at the time for what it called a failure of leadership in acting to end the two-year war between government forces and rebels that has killed tens of thousands in the world's youngest country.
Kerry spoke with Kiir by telephone yesterday, State Department spokesman John Kirby said.
The head of the rebel group, Riek Machar, signed the peace accord Monday, and on Tuesday the US and Britain pushed for UN sanctions against Kiir if he fails to sign it.
Both the government and rebels accused the other side of launching new attacks yesterday.
