The White House sharply criticized the renewed heavy fighting in South Sudan's capital city Juba that has pushed thousands of people to flee.
"The United States condemns in the strongest terms the return to violence in South Sudan. It must stop," National Security Advisor Susan Rice said in a statement.
"This senseless and inexcusable violence — undertaken by those who yet again are putting self-interest above the well-being of their country and people — puts at risk everything the South Sudanese people have aspired to over the past five years."
The United States has long been a key supporter of South Sudan, the world's youngest nation, which became independent from Sudan in 2011.
South Sudan's President Salva Kiir and his estranged deputy Riek Machar ordered a ceasefire, kindling hope that three days of fierce fighting between the army and former rebels would end.