Power said the U.S. government welcomed the signing of an agreement to end hostilities between warring parties in South Sudan. The agreement was reached in Ethiopia by representatives of South Sudanese President Salva Kiir and former Vice President Riek Machar.
"More innocent lives are being lost every day this conflict goes on," she said in a statement Thursday. "We call on both parties to abide by the cessation of hostilities agreement, cooperate on addressing root causes of the conflict."
Violence erupted in South Sudan in mid-December after Kiir accused the former vice president of trying to overthrow the government. Machar, sidelined when Kiir reshuffled his Cabinet in July, denies attempting a coup.
Both leaders are from rival ethnic communities in South Sudan.
The White House issued a statement Thursday saying South Sudanese leaders should honor the terms of the agreement in order regain the trust of South Sudan's people and members of the international community.
The International Crisis Group estimates at least 10,000 people died as a result of the violence.
The U.S. government helped broker regional agreements that secured South Sudan's independence in 2011. The truce in Ethiopia was steered in part by the regional Intergovernmental Authority on Development.