South Sudan's President Salva Kiir declared an overnight curfew in the capital after the government foiled what he called an attempted coup.
Mr. Kiir told reporters Monday that the government is in "full control" of the security situation in the capital, Juba, after an attack at the army headquarters.
Dressed in his military uniform rather than his usual civilian clothes, Mr. Kiir said the attack was carried out by forces loyal to his former vice president, Riek Machar. Machar was fired by the president in July.
Witnesses in Juba said they heard heavy gunfire and explosions overnight Sunday into Monday morning but that the fighting mostly subsided by Monday afternoon.
The U.S. Special Envoy to Sudan and South Sudan, Donald Booth, told VOA that the United States is very concerned about the developments. He said the United States cannot yet confirm a coup attempt and are trying to learn what sparked the violence.
"The situation remains a bit confused. The embassy in Juba has not been able to get out much due to the fighting around town. Right now they are sheltering in place until they feel it's safe to move around the city."
International flights in and out of the capital have been canceled.
The U.S. State Department described the situation as very fluid and called on all parties to resolve their differences peacefully. Spokeswoman Marie Harf said that given the history of conflict in the region, the United States is concerned that the violence could spread.
U.N. Secretary General Ban Ki-moon said he is deeply concerned by the fighting and what he said was "the risk of targeted violence against certain communities." He said the government must guarantee the security of all civilians regardless of which community they come from.
Hilde Johnson, the U.N. special representative for South Sudan, urged everyone involved to end the fighting and said she had been in contact with key leaders to call for calm.
The U.N. Mission in South Sudan said in a statement that hundreds of civilians have sought refuge at its compound and denied "any suggestion" that it is harboring political or military figures.
In a statement, the U.S. embassy also said that "no political or military figures" had taken refuge within its walls.