"Now is the time for South Sudan's leaders to rein in armed groups under their control, immediately cease attacks on civilians, and end the chain of retributive violence between different ethnic and political groups," Kerry said, as he announced plans to dispatch Ambassador Donald Booth, his special envoy for Sudan and South Sudan, to the region.
Kerry added that the decision, which came as battles intensify between rival ethnic groups across South Sudan, followed a phone call he made Thursday to South Sudan's President Salva Kiir.
"Last night, I called South Sudanese President Kiir and urged him, as president of all of South Sudan, to protect all South Sudanese citizens and work toward reconciliation," the US top diplomat said.
"We recalled the difficult decisions that led to the remarkable moment when so many stood in long lines for a referendum to give birth to South Sudan, knowing all too well that the toughest decisions were still to come. Now is the time for leadership that makes those decisions through dialogue," Kerry said.
South Sudan, the world's newest country, split from Sudan in July 2011 after an independence referendum under a peace deal that ended a 22-year civil war.
US National Security Adviser Susan Rice said in a message to the people of South Sudan on Friday that "if individuals or groups seek to take or hold power through force, mass violence, or intimidation, the United States will have no choice but to withdraw our traditional, robust support.
"We urge leaders on all sides to publicly renounce violence, end the fighting, and commit to peaceful dialogue. Ethnic violence must cease immediately. Those who have committed acts of violence against civilians must be held accountable," Rice said.
The message comes nearly a week into violent clashes between followers of Kiir, an ethnic Dinka, and his former vice president Riek Machar, a Nuer, that have killed at least 500 people in the capital Juba alone.
One of the attacks targeted a United Nations base, leaving at least 11 civilians and two Indian peacekeepers dead.
UN officials reported that up to 3,000 armed youths had gathered around another camp at Bor in Jonglei state where 14,000 people have sought refuge.