The world's newest nation is reeling from four consecutive years of flooding, with the disaster now affecting nine out of ten states, the Office for the Coordination of Humanitarian Affairs (OCHA) said in a briefing note.

"Reportedly, the floods destroyed livestock and crops; washed away roads and bridges; destroyed homes, schools and health facilities; and submerged boreholes and latrines thereby contaminating water sources and risking outbreaks of waterborne diseases," OCHA said.

In oil-rich Unity state, one of the worst-hit regions, rising water levels breached dykes in two places on Sunday, threatening to flood camps for internally displaced people as well as a base for the UN Mission in South Sudan (UNMISS).

"Efforts are ongoing around the clock to repair the areas needed and to monitor any vulnerable areas ahead of further breaches," OCHA said.

In Western Bahr el Ghazal state, heavy rains caused a key bridge to collapse, cutting off critical aid deliveries to already struggling populations, OCHA said.

In its previous update released last month, the UN agency had estimated that around 386,000 people were affected by flooding across seven states.

Four out of five of South Sudan's 11 million people live in "absolute poverty," according to World Bank figures for 2018, and nearly two-thirds of its population suffer from severe hunger.

Since achieving independence from Sudan in 2011, the young nation has been in the throes of a chronic economic and political crisis, and is struggling to recover from the aftermath of a five-year civil war that left nearly 400,000 people dead.

Although a 2018 ceasefire and power-sharing deal between President Salva Kiir and his deputy Riek Machar still largely holds, little progress has been made in fulfilling its terms.

One of the poorest countries on the planet despite large oil reserves, South Sudan's leadership has faced fierce criticism from the UN for failing its people and stoking violence.