The president of neighbouring Sudan is also said to be in the city as regional player continue to call for a truce in fighting between the government and rebels loyal to former vice president Riek Machar.
Gunfire lasting about an hour rang out in the northern part of Juba and came from the military headquarters of the government, Reuters reported.
The current disorder in South Sudan – the world’s newest nation – began in Juba in mid-December. Fighting was soon quelled but spread to neighbouring states, where rebels have at various times seized key towns and oil patches.
President Salva Kiir has accused Machar of plotting a coup to overthrow him, something the latter has denied.
Regional leaders have called for an end to the fighting and peace talks were to begin in the Ethiopian capital of Addis Ababa almost a week ago. While negotiators from both sides have met with mediators, it appears the warring factions have not actually held face-to-face meetings as yet.
The impasse has led President Omar Bashir of Sudan to fly down to Juba with aides to hold talks with the government.
South Sudan only split with Sudan in July 2011, taking with it three-quarters of the former country’s oil reserves. The neighbours have been at loggerheads virtually ever since.