Alastair Leithead says the majority of the refugees are from the Dinka community
Direct talks aimed at ending weeks of violence in South Sudan are due to begin.
Ethiopia's Foreign Affairs Minister Tedros Adhanom said the initial meetings had been "fruitful".
Continued fighting between supporters of President Salva Kiir and his sacked deputy, Riek Machar, has killed at least 1,000 people since 15 December.
The US has said it remains committed to ending the violence, despite evacuating most of its embassy staff from Juba.
"Even as we draw down our personnel, the United States remains deeply and actively committed to supporting regional and international efforts to end the violence in South Sudan," said State Department deputy spokeswoman Marie Harf.
More than 180,000 people have been displaced in the conflict
US embassy staff boarded a KC-130 aircraft to leave Juba
Mabior Garang, left, part of Riek Machar's delegation, arrives in Addis Ababa
The violence has been continuing, with reports of heavy fighting on the outskirts of the rebel-held city of Bor, in Jonglei state, on Friday,
The government said its troops were attempting to recapture both Bor and the city of Bentiu, in the northern state of Unity. At the same time, rebel forces said they were advancing on the capital, Juba.
Ceasefire
Delegates from both sides began arriving in the Ethiopian capital on Wednesday but talks were delayed until the full negotiating teams had arrived.
The BBC's Emmanuel Igunza in Addis Ababa says the rival teams were in the same hotel but on Friday had only held talks with mediators, who were preparing the ground for direct negotiations.
The Ethiopian minister of foreign affairs said face-to-face talks would follow.
"We just finished the first round of proxy talks with both negotiating teams of #SouthSudan. Will proceed to direct talks tomorrow," Mr Adhanom said on Twitter on Friday.
Observers have said the discussions are likely to be complicated, as the two sides will have to agree on a mechanism to monitor any ceasefire.
It flew out a large number of non-essential staff soon after the fighting began on 15 December.
More than 440 US officials and citizens have been evacuated on charter flights and military aircraft, Reuters quoted the State Department spokeswoman as saying.
The Pentagon sent two KC-130 aircraft to pick up about 20 US diplomatic personnel from the embassy in Juba on Friday, said Army Colonel Steve Warren.
Aid effort
Ambassador Susan Page said on Twitter: "We are not suspending our operations. We are just minimising our presence."
However, the state department also said that, from Saturday, it would no longer be providing consular services to US citizens in South Sudan.
The UN, however, is flying more staff into Juba to help in the aid effort and to protect civilians' human rights. One official said US staff working for the UN had not been asked to leave.
South Sudan is the world's newest state. It was formed in 2011, gaining independence from Sudan after decades of conflict.
The latest trouble has its roots in tensions that go back long before 2011, rebels were fighting each other as well as for independence.
But what began as a squabble between former fighters turned politicians has taken on an ethnic dimension.
Politicians' political bases are often ethnic. President Kiir is from the Dinka community while Mr Machar is a Nuer.
Mr Kiir has ruled out any power-sharing arrangement with his rival in the longer term.
Fighting erupted in the South Sudan capital, Juba, in mid-December. It followed a political power struggle between President Salva Kiir and his ex-deputy Riek Machar. The squabble has taken on an ethnic dimension as politicians' political bases are often ethnic.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
Both Sudan and the South are reliant on oil revenue, which accounts for 98% of South Sudan's budget. They have fiercely disagreed over how to divide the oil wealth of the former united state - at one time production was shutdown for more than a year. Some 75% of the oil lies in the South but all the pipelines run north
The two Sudans are very different geographically. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
After gaining independence in 2011, South Sudan is the world's newest country - and one of its poorest. Figures from 2010 show some 69% of households now have access to clean water - up from 48% in 2006. However, just 2% of households have water on the premises.
Just 29% of children attend primary school in South Sudan - however this is also an improvement on the 16% recorded in 2006. About 32% of primary-age boys attend, while just 25% of girls do. Overall, 64% of children who begin primary school reach the last grade.
Almost 28% of children under the age of five in South Sudan are moderately or severely underweight - this compares with the 33% recorded in 2006. Unity state has the highest proportion of children suffering malnourishment (46%), while Central Equatoria has the lowest (17%).
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