South Sudan is "back to war", a spokesman for the vice-president has told the BBC, as rebel factions clash.
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.