Psaki said there may be serious consequences if the terms of a January truce are violated.
"We are deeply concerned by reports of violations by both the government of South Sudan and anti-government forces of the cessation of hostilities agreement that was signed in Addis Ababa [Ethiopia's capital] on Jan. 23," she said in a statement Saturday.
Conflict erupted in South Sudan last month after President Salva Kiir accused former Vice President Riek Machar of trying to overthrow the government. Machar, sidelined when Kiir reshuffled his Cabinet in July, denies attempting a coup.
Machar told Voice of America last week he has formed a resistance movement against Kiir's administration.
He said he was tired of war.
"We want our people to develop and catch up with the rest of the world," Machar was quoted as saying.
The U.S. government helped broker a 2005 peace agreement that ended civil war in Sudan and opened the way to South Sudan's independence in 2012.