South Sudanese President Salva Kiir declared a curfew in Juba, accusing soldiers allied with the former vice president of attempting a coup overnight in the capital.
In a press conference in the capital Monday, Kiir said a group of soldiers allied with former Vice President Riek Machar launched an attack late Sunday at army headquarters.
Witnesses reported hearing heavy bouts of gunfire and explosions in the capital into Monday morning.
The president said the government now has the situation under control.
"I would like to inform you all, my dear citizens, that your government is in full control of the security situation in Juba," Kiir said. "The attackers fled and your armed forces are pursuing them. I promise you today that justice will prevail."
Machar, who has announced his ambitions to run for president, was dismissed from his position in July as part of an overall Cabinet reshuffle.
An outspoken critic of the president, Machar has in the recent past accused Kiir of behaving like a dictator.
Neither Machar, nor his spokesperson, could be immediately reached for comment.
In response to the fighting, Kiir has declared a nighttime curfew in Juba until further notice.
International flights in and out of the capital have been cancelled.
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.