An eye witness saw heavily armed soldiers patrolling the streets of Juba early today amid sporadic gunfire emerging from Juba's main army barracks.
It was not possible to get a comment from official government or military spokespeople because of the poor telephone network.
There has been political tension in the world's youngest nation since South Sudan President Salva Kir sacked Riek Machar as his deputy in July.
Machar, who has expressed a willingness to contest the presidency in 2015, said after his sacking that if the country is to be united it cannot tolerate a "one man's rule or it cannot tolerate dictatorship."
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The local Sudan Tribune newspaper reported on its website that clashes erupted late yesterday between members of the presidential guard in fighting that seemed to pit soldiers from Kiira's Dinka tribe against those from the Nuer tribe of Machar.
In a message to American citizens today, the US Embassy in Juba said it had received "reports from multiple reliable sources of ongoing security incidents and sporadic gunfire in multiple locations" across Juba.
"The US Embassy has not been able to confirm that gunfire and insecurity have fully ceased," the message said.
Hilde Johnson, special representative of the United Nations secretary-general for South Sudan, said in a statement that the UN mission in Juba was "deeply concerned" over the fighting that broke out late yesterday and which continued today.
"As the Special Representative of the Secretary General I urge all parties in the fighting to cease hostilities immediately and exercise restraint," the statement sad.