Thousands of civilians have fled a state capital in South Sudan, amid fears of new clashes between rebels and government soldiers.
U.N. humanitarian official Toby Lanzer visited Bentiu on Thursday. In a series of Twitter messages, he said he saw "virtually no civilians" in the center of town, and that shops in Bentiu's main market have been looted and largely destroyed.
Reports from the area say army troops loyal to President Salva Kiir are advancing toward the town, which has been held for several weeks by rebels who support the president's rival, Riek Machar.
Peace talks between the sides in Ethiopia remained at an impasse Thursday, with the government again refusing to release 11 political detainees, as demanded by the rebels.
In remarks to the U.S. Senate Foreign Relations Committee, the top U.S. diplomat for Africa, Linda Thomas-Greenfield, said the U.S. is strongly urging a peaceful resolution to the conflict.
"This crisis will not be solved on the battlefield, and we have made that point over and over again. Although fighting started less than one month ago, the roots of this conflict are much deeper, and resolution can only come from immediate dialogue between the two sides and a broader reconciliation."
More than 1,000 people have been killed in South Sudan since violence began in mid-December. Some 200,000 have been displaced.
The United Nations says more than 60,000 are sheltering on U.N. bases throughout the country, including 8,000 in Bentiu.
On Thursday, U.N. Emergency Relief Coordinator Valerie Amos said the U.N. is releasing $15 million to support relief efforts in South Sudan. She said U.N. agencies will use the funds to improve living conditions of people stuck in overcrowded camps.
South Sudan's unrest began December 15 with fighting at an army headquarters in the capital, Juba. President Kiir accused his former vice president, Machar, of a coup attempt.
Many soldiers aligned themselves with Machar and seized control of Bentiu and the Jonglei state capital, Bor. Heavy fighting has been reported this week in several parts of the country.
Witnesses say the recent violence has an ethnic component, with targeted attacks between supporters of Mr. Kiir, from the Dinka tribe, and Machar, who is from the Nuer community.