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A Reuters reporter heard gunfire coming from the direction of the military headquarters of the SPLA government forces, towards the northern outskirts of the city.
It lasted for about an hour before dying down.
South Sudan has been rocked by fighting that erupted last month in the capital but quickly spread to other parts of the country.
Although Juba has been largely calm since the first days of the unrest, there was a brief gun fight on Saturday evening, and residents talk of growing tensions.
"I saw a truck full of soldiers going along the Bilpam road. They were singing. About 20 minutes later the shooting started and people started running towards town," said Animu Afekuru, who lives in the neighbourhood.
Western and regional powers, many of which supported the negotiations that led to South Sudan's secession from Sudan in 2011, are pressing for a peace deal, fearing the latest fighting could spiral into civil war and destabilise east Africa.
Rebel and government negotiators were supposed to begin their first face-to-face talks at 3pm (local time) in Addis Ababa with the aim of halting the clashes, which have killed more than a thousand people. But by 8:15pm there was no sign of the two sides sitting down together.
The talks were first set for Jan. 1. Several false starts have dampened hopes for a swift end to the fighting between President Salva Kiir's SPLA government forces and rebels loyal to former vice president Riek Machar.
Both warring factions have said they want peace and are committed to a ceasefire in principle, though neither has indicated when they would lay down their weapons.
But there's widespread scepticism in Juba, where residents are on edge amid swirling rumours of a rebel advance on the city that lies on the banks of the White Nile.
"I fear for our country in the coming days," said 19-year-old Nyathok Khat before Sunday's gunfire. "The politicians don't care about the suffering of the people."
The unrest, which has forced South Sudan to cut its oil output, has driven more than 200,000 people from their homes. The United Nations is scrambling to raise money to provide food, clean water and shelter.