Forces loyal to the vice-president of South Sudan say their positions in the capital, Juba, have been attacked by government troops.
UN representatives reported heavy exchanges of gunfire near their headquarters in the suburb of Jebel.
The clashes follow days of fighting between supporters of President Salva Kiir and Vice-President Riek Machar.
The violence has raised fears of renewed instability, with a 2015 peace deal failing to quell unrest.
Heavy gunfire was reported on Sunday in the Gudele and Jebel suburbs of Juba, near a military barracks occupied by troops loyal to Mr Machar.
"For about 30 to 40 minutes we heard sounds of heavy artillery in the direction of Jebel area," an aid worker based in Juba told Reuters.