"Floods have caused extensive damage to homes, crops and critical infrastructure, disrupting education and health services and increasing the risk of disease outbreaks," said UN humanitarian agency OCHA in a situation report on Thursday.

It said that as of September 5 "flooding has affected more than 710,000 people across 30 of 78 counties".

Since gaining independence in 2011, the world's newest nation has remained plagued by instability and violence, despite rich oil reserves.

Aid agencies have warned the country is facing its worst flooding in decades and OCHA said aid is failing to reach many of those in need.

"Access remains a critical obstacle, as many roads are now impassable, and floodwaters have cut off entire communities," it said.

Plagued by feuding leaders and chronic instability, some 400,000 people died and millions were displaced in a civil war between 2013 and 2018.

President Salva Kiir and his rival Riek Machar signed a peace deal in 2018 and formed a unity government.

But the country has since battled natural disasters, hunger, violence and continued political in-fighting that has repeatedly delayed promised elections.