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.

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.

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.