Forces loyal to Vice-President Riek Machar say their positions in the capital, Juba, have been attacked by government troops.
Col William Gatjiath, Mr Machar’s military spokesman, told the BBC President Salva Kiir “isn’t serious” about a peace agreement.
The government has not responded to Col Gatjiath’s comments.
He
said “hundreds” of Mr Machar’s troops had died on Sunday, and that
troops loyal to Mr Machar were advancing on Juba from different
directions.
UN representatives have reported heavy exchanges of gunfire near their headquarters in the suburb of Jebel.
A tweet by the UN’s official mission in South Sudan, posted on Sunday afternoon, said fighting was now “relentless”.
The
clashes follow days of fighting in the world’s newest country between
supporters of Mr Kiir and Mr Machar. At least 150 people were killed in
fighting on Friday, before calm was restored on Saturday.
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.