Three years years after the people of South Sudan turned out in huge numbers to vote for independence from Sudan, the world's newest nation is standing on the brink of failure, a senior U.S. official told lawmakers on Capitol Hill Thursday.

"On January 9, 2011, the people of South Sudan voted in overwhelming numbers for independence from the Republic of Sudan," Assistant Secretary of State Linda Thomas-Greenfield told a Senate Foreign Relations Committee hearing on the situation in South Sudan.

"After decades of war, they were peacefuilly and joyfully voting for separation and a new future. Today, tragically, the world's youngest country and undoubtedly one of the most fragile democracies is in danger of shattering," she said.

As Thomas-Greenfield spoke in Washington, talks aimed at bringing peace to South Sudan were on hold in Addis Ababa after mediators from the Intergovernmental Authority on Development (IGAD) failed during a trip to Juba to convince South Sudanese President Salva Kiir to release 11 senior members of the ruling party who were arrested when the unrest started on Dec. 15.

Kiir has said the unrest was triggered by an attempted coup, led by his former Vice President Riek Machar. Machar has consistently denied the accusations, and observers have said the violence erupted when renegade soldiers attacked an army headquarters building in Juba.

Delegates at the talks for Machar have asked that the detainees be released before they will agree to a ceasefire, but Kiir "refused point-blank" to the IGAD mediators' requrest to release them, a spokesman for Machar said Thursday.

In spite of the negotiations stalling, they were not in danger of collapsing, the spokesman, Hussein Mar Nyuot, told reporters in Addis Ababa Thursday.

"We are disappointed... that the president turned down the request to release the political detainees but it is something we can discuss," Mar Nyuot said.

"I can't say the talks are going to collapse...On our part we are very positive. We are not pulling out of IGAD," he said, adding that "big pressure" is being put on Kiir "and we are hopeful he will change his mind" and release the detainees.

Mar Nyuot said the talks will resume when IGAD mediators call both sides back to the negotiating table.

Civilians Flee Bentiu

On the ground in South Sudan, civilians continue to flee fighting between government troops and forces loyal to Machar.

United Nations' humanitarian official Toby Lanzer described in a series of Twitter messages scenes of devastation and violence in the town of Bentiu, in Unity State, which fell to forces loyal to Machar days after the fighting erupted in Juba more than three weeks ago.

Many people were "on the move," Lanzer tweeted, describing Bentiu as a ghost-town with "virtually no civilians" amid reports that government troops are advancing toward the town.

The market, usually a hive of activity and commerce, has been destroyed, and heavily armed men who say they back Machar are walking around the city, Lanzer wrote.

The U.N. Mission in South Sudan (UNMISS) has evacuated some 500 civilians who had gathered at the compound of the World Food Programme in Bentiu and taken them to the U.N. base for safety.

Mongolian peacekeepers have dug a trench and built a berm around the base to try to better protect the civilians sheltering there, he said.

The United Nations says more than 60,000 are sheltering on U.N. bases throughout the country, including 8,000 in Bentiu.

Some 250,000 people have been displaced by the fighting, and while no official death toll has been released, a top U.N. official said Thursday it is likely to be "very substantially in excess of the figure of 1,000 that we know for sure about."