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.
