South Sudanese President Salva Kiir says he is willing to hold talks with the former vice president he accuses of leading a coup attempt against him.
The United Nations estimates four days of fighting in South Sudan have killed up to 500 people.
U.N. Secretary-General Ban Ki-moon told reporters Wednesday he has urged President Kiir to hold talks with his opponents and cooperate with the U.N. during this volatile time.
"This is a political crisis, and urgently needs to be dealt with through political dialogue. There is a risk of this violence spreading to other states, and we have already seen some signs of this."
Mr. Ban said as many as 20,000 people have take refuge with the U.N. mission in the capital, Juba. South Sudanese officials have urged residents to return to their homes following the deadly clashes.
The International Committee of the Red Cross, however, says many civilians are "too scared to go home."
ICRC spokesperson Cynthia Lee told VOA the two main hospitals in Juba are overstretched as they deal with the influx of fighting casualties. She said her group has delivered wound dressing materials and medicine to treat the victims at the hospitals.
The Sudan People's Liberation Army has called all soldiers to report to their general headquarters.
President Salva Kiir blames the alleged coup attempt on forces loyal to his former vice president, Riek Machar, who was fired in July.
Observers have raised concerns a rift between Machar, from the Nuer ethnic group, and Mr. Kiir, a Dinka, could fuel tribal violence in South Sudan.
South Sudan's government said 10 people have been arrested for their alleged roles in the coup attempt. Machar remains at large.
Britain says it is withdrawing some embassy staff from South Sudan after the fighting spread to areas beyond the capital.
The British Foreign Office says its embassy in Juba will remain open while some staff is withdrawn temporarily.
The U.N. mission in South Sudan has reported heavy fighting in the city of Bor, about 150 kilometers north of Juba.
There was also fighting overnight at a military base in Torit, southeast of the capital.
U.S. Secretary of State John Kerry says the people of South Sudan have sacrificed too much for their country to return to violence.
"Political differences need to be resolved by peaceful and democratic means and those have been hard fought for. The government should respect the rule of law and the people of South Sudan should be able to realize their full potential in peace."
The U.S. State Department says it has evacuated three groups of American citizens from South Sudan. The U.S. has organized at least one other evacuation for Thursday.
Ambassador Susan Page met Wednesday with President Kiir in Juba to discuss U.S. concerns about the continued violence, increasing death toll and growing humanitarian challenges.