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 on Thursday visited Bentiu, the capital of Unity state. In a series of Twitter messages, he said he saw "virtually no civilians" in the center of town, and said shops in the 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.
Meanwhile, peace talks in neighboring Ethiopia remain at an impasse, with the government again refusing rebel demands for the release of 11 political detainees.
A rebel spokesman at the Addis Ababa talks used the stalemate Thursday to accuse Ugandan forces and gunships of attacking rebel positions. However, Uganda claims its military presence in South Sudan is limited to protecting its stranded countrymen.
For his part, the lead Sudanese government negotiator dismissed the rebel claims of Ugandan aerial attacks as "hostile propaganda."
The top U.S. diplomat for Africa, Linda Thomas-Greenfield, told the U.S. Senate Foreign Relations Committee that U.S. officials are 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, when President Kiir accused his former vice president, Machar, of orchestrating a coup attempt. The United Nations says about 200,000 people have been displaced, with another 60,000 sheltering on U.N. bases throughout the country.
On Thursday, U.N. Emergency Relief Coordinator Valerie Amos said the world body is releasing $15 million to support relief efforts for refugees stuck in overcrowded camps.
Witnesses say the 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.