South Sudan refugees reach one million mark
More than 1.6 million people are also displaced within South Sudan, meaning about 20% of the population have been made homeless since December 2013.
A fragile peace deal signed last year is on the brink of collapse.
"The violence in July came as a major setback to peace efforts in South Sudan," the UNHCR spokesman Leo Dobbs said in a statement.
The UN says more than 185,000 people have fled South Sudan since July.
"The fighting has shattered hopes for a real breakthrough and triggered new waves of displacement and suffering, while humanitarian organisations are finding it very difficult for logistical, security and funding reasons to provide urgent protection and assistance to the hundreds of thousands in need," Mr Dobbs said.