Ugandan soldiers have flown to South Sudan to help evacuate their citizens, an army spokesman said on Friday, although two military sources said the troops would also help secure the capital, which lies about 75 km (50 miles) from Uganda's border.
Uganda has said it is worried by fighting that erupted this week and which threatens to plunge the new nation into an ethnic war.
Uganda backed the SPLM/SPLA insurgency led by Salva Kiir - now president of South Sudan - before it won independence in 2011.
Like other neighbors, Uganda hosted many refugees from the decades of civil war in pre-partition Sudan and worries about the two-year-old nation next door collapsing into chaos.
“We have military personnel in Juba but they are there strictly to help rescue and evacuate stranded Ugandans, some of whom are injured, and our personnel are there at the request of South Sudan government,” army spokesman Paddy Ankunda said.
“We're not involved in any military activity there.”
He denied a report in a state-owned newspaper that the troops would also help secure Juba. But two military sources, who spoke on condition of anonymity, told Reuters securing the capital would be part of the mission.
“Some troops from Special Forces Command - I can estimate in hundreds - left for Juba yesterday,” said a source in the Special Forces Command, a unit led by Ugandan President Yoweri Museveni's son.
“They will mainly be involved in securing the capital,” he said, adding that some had gone by plane while others were expected to travel by road.
“They're not going to participate in the skirmishes between Kiir and Machar,” he said, referring to South Sudan's former vice president Riek Machar.
Kiir, from the dominant Dinka ethnic group, has accused Machar, a Nuer, of attempting to stage a coup. The two ethnic groups have clashed in the past.
Uganda's minister of state for international affairs, Okello Henry Oryem, joined a mission of African ministers and other mediators who held talks with Kiir in Juba on Friday to try to broker peace.