Radio Erena:- December 16, 2013
Local media said the fighting was between rival factions of the presidential guard and focused around their military barracks.
Heavily armed troops are now patrolling Juba, and army spokesman Col Phillip Aguer told the BBC the military was in full control.
The UN expressed concern and appealed for all sides to show restraint.
President Salva Kiir is expected to make a statement shortly.
South Sudan formally split from Sudan in 2011, after decades of conflict. But the oil-rich country is ethnically and politically divided, with many armed groups active.
Tensions have been particularly high since President Kiir dismissed his entire cabinet, including his deputy Riek Machar, in July in an apparent power struggle.
Mr Machar had indicated he planned to contest the presidential elections in 2015.
Mr Kiir is from the Dinka community, which is the largest in South Sudan, while Mr Machar is from the Nuer, the second-largest. Some Nuer have complained about Dinka political domination.
The fighting in Juba reportedly broke out overnight, and intensified in the early morning.
The Paris-based Sudan Tribune said the clashes began when one mostly Nuer unit of the presidential guard became suspicious of deployments of a group of mainly Dinka guards.
There were reports of continuous gunfire and the sound of explosions.
State TV channel SSTV was off air and the city’s airport has been closed.
The situation had reportedly calmed by mid-morning, but heavily armed troops were seen on the capital’s streets.
One resident who lives near the presidential guard barracks told the BBC that many people had sought refuge at a Catholic church.
Col Aguer said the army was “establishing the facts about the identity of those who started the fighting”.
“The military intelligence is gathering information. As soon as the situation is cleared, the government will come up and the army will make a statement about what it was,” he said.
Hilde Johnson, the UN’s special representative in South Sudan, said she was “deeply concerned” about the fighting and urged “all parties in the fighting to cease hostilities immediately and exercise restraint”.
“I have been in touch regularly with the key leaders, including at the highest levels to call for calm,” she said.
The UN mission in Juba said earlier that staff there were under lockdown.
The UK and US embassies in Juba urged their citizens via Twitter to stay indoors and exercise caution.
The US statement to citizens said it had suspended all routine services amid “reports from multiple reliable sources of ongoing security incidents and sporadic gunfire in multiple locations across Juba”.
The US embassy also denied rumours that Mr Machar had taken shelter there.
In a second statement, the US said embassy staff had spoken to a range of officials and concerned parties “in order to urge calm, restraint, and a settling of differences through a peaceful political means rather than through violence”.