The recent violence in Juba echoed the fighting that triggered the civil war and marks a fresh blow to last year's peace deal to end the bitter conflict that began when President Salva Kiir accused ex-rebel and now Vice President Riek Machar of plotting a coup.
"It's over 300 deaths since August 8," said World Health Organisation spokesman Tarik Jasarevic. The UN however said it did not have the number of injured.
"The number of refugees in neigbouring countries is now 835,000," he said.
However, the International Organisation for Migration said many people were returning.
Also Read
"Humanitarian access to affected people has improved dramatically since Monday. But this can only be sustained if the ceasefire holds", said John McCue, IOM South Sudan Head of Operations.
Machar's sacking as vice-president in 2013 set off a cycle of retaliatory killings that split the poverty-stricken, landlocked country along ethnic lines and drove more than two million out of their homes.
According to the UN, there were some 114,000 South Sudanese refugees in neighbouring countries before December 2013 but that figure has ballooned to 835,000 now.