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 Organization spokesman Tarik Jasarevic. The UN however said it did not have the number of injured.
The July 8-11 violence had left "42,000 internally displaced" in the world's youngest nation, said William Spindler, the spokesman for the UN refugee agency.
"The number of refugees in neighboring countries is now 835,000," he said, AFP reported.
However, the International Organization for Migration said many people were returning.
"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.
The conflict has been characterized by horrific rights abuses, including gang rapes, the wholesale burning of villages and cannibalism.
According to the UN, there were some 114,000 South Sudanese refugees in neighboring countries before December 2013 but that figure has ballooned to 835,000 now.