Uganda has been a long-time military supporter of South Sudan's President Salva Kiir and announced in March that it had deployed special forces to the country.

It was backing Kiir as he moved against his long-time rival, Riek Machar, and militias from Machar's ethnic Nuer community.

But the clashes on Monday appeared to be between the Ugandan People's Defence Forces (UPDF) and a local detachment of government troops in Central Equatoria State, near their shared border.

"Elements from the armies of two sisterly Republics of South Sudan and Uganda exchanged fires" in Kajo Keji County, the South Sudan army said in a statement.

It said the chief of its defence staff had spoken with his Ugandan counterpart to ensure hostilities ceased and launch an investigation into how the clashes began.

A statement by Kajo Keji County representatives said the "attack by the UPDF... has resulted in loss of lives and injuries from both sides".

The representatives said thousands were forced to flee their homes and were "sheltering in bushes, religious premises and local schools".

Uganda sent troops to support Kiir when civil war broke out in the country in 2013, just two years after it had gained independence from Sudan.

The war between Kiir and Machar lasted five years and left some 400,000 dead before a power-sharing agreement was reached in 2018.

That deal has been all but buried by Kiir's recent moves to sideline Machar, raising fears of renewed ethnic conflict.

The Ugandan army has been accused of using chemical weapons, namely barrel bombs containing a flammable liquid that killed civilians, against Nuer militias in South Sudan's northeast.

Uganda has denied the accusations.