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, but South Sudan's information minister denied the presence of troops, as tensions between South Sudan's president and first vice president stoke fears of a return to 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.

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.

Uganda's military chief said Tuesday his country had deployed special forces in South Sudan's capital Juba to "secure it" as tensions between President Salva Kiir and his First Vice President Riek Machar stoke fears of a return to civil war.