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.

Tensions have been growing in recent days in South Sudan, an oil producer, after Kiir's government detained two ministers and several senior military officials allied with Machar. One minister has since been released.

The arrests in Juba and deadly clashes around the northern town of Nasir are seen as jeopardising a 2018 peace deal that ended a five-year civil war between forces loyal to Kiir and Machar that cost nearly 400 000 lives.

"As of 2 days ago, our Special Forces units entered Juba to secure it," Uganda's military chief, Muhoozi Kainerugaba, said in a series of posts on the X platform overnight into Tuesday.

"We the UPDF (Ugandan military), only recognise one President of South Sudan, H.E. Salva Kiir ... any move against him is a declaration of war against Uganda," he said in another post.

South Sudan government information minister and the military spokesperson did not pick up phone calls seeking comment.

After the civil war erupted in South Sudan in 2013, Uganda deployed its troops in Juba to bolster Kiir's forces against Machar. They were eventually withdrawn in 2015.

Ugandan troops were again deployed in Juba in 2016 after fighting reignited between the two sides but they were also eventually withdrawn.

Uganda fears a full-blown conflagration in its northern neighbour could send waves of refugees across the border and potentially create instability.

Kainerugaba did not say whether the latest deployment was in response to a request from Kiir's government or how long the troops would remain in South Sudan.