Uganda has deployed special forces in South Sudan's capital Juba to "secure it", Uganda's military chief said on Tuesday, as tensions between South Sudan's president and first vice president stoke fears of a return to civil war.

Uganda has deployed an unknown number of troops to South Sudan in a bid to protect the fragile government of President Salva Kiir as a tense rivalry with his deputy threatens a return to civil war in the east African nation.

Tension mounts in South Sudan as power deal between President Salva Kiir and Vice President Riek Machar unravels.

The situation in South Sudan is fragile and getting worse, with growing political and military instability. The claims made by the information minister do not reflect the reality on the ground.

South Sudan's government has denied earlier reports that Uganda had deployed special forces to its capital, Juba. Uganda's military chief Gen Muhoozi Kainerugaba said the soldiers had gone to the neighbouring country to help South Sudanese President Salva Kiir "secure" the city.