Power said in a statement posted on her official Twitter account that an attack this week on the U.N. Mission in South Sudan was unacceptable.
"[The United States] will work within the U.N. Security Council to hold accountable those who attack UNMISS bases," she said late Thursday.
The U.N. mission reported Tuesday at least 10 people were killed at its hospital in Upper Nile state when fighting broke out between pro- and anti-government forces. UNMISS said in a statement the conflict "will only exacerbate an already dire situation" in South Sudan.
Conflict erupted in December when South Sudanese President Salva Kiir, from the Dinka ethnic community, accused former Vice President Riek Machar, from the rival Nuer community, of trying to overthrow the government. Machar was sidelined when Kiir reshuffled his Cabinet in July.
Power in her statement said the United States "utterly" condemns the lingering violence
The U.S. government helped broker the 2005 truce that ended Sudan's civil war and paved the way to South Sudan's independence in 2012. The U.S. Senate Foreign Relations committee scheduled a hearing Feb. 26 to address the crisis.