South Sudan celebrated its second anniversary of independence from Sudan Tuesday. Independence was ensured through a 2005 peace deal with Sudan, brokered with the help of the U.S. government.
At least 2 million people died as a result of civil war in Sudan.
Kerry said Tuesday referendums do not equate to sustained democracies, nor do they provide guarantees for good governance and peace.
"The vision that South Sudan laid out for itself two years ago requires a sustained commitment to democracy and good governance, justice and accountability, and respect for the rule of law and the human rights of all of South Sudan's people," he said in a statement.
Border conflicts, ethnic fighting and financial disputes with Sudan have threatened the peace deal securing South Sudan's independence.
Hilde Johnson, U.N. special envoy for South Sudan, said Monday from South Sudan the country has experienced "many setbacks and problems" since independence in 2011.
A group of human rights activists sent a letter to South Sudanese President Salva Kiir expressing concern about the country's long-term prospects.
"We have all come to conclude that without significant changes and reform, your country may slide toward instability, conflict and a protracted governance crisis," the letter said.