Extreme floods across vast parts of remote and impoverished South Sudan have affected over 156,000 people, the United Nations said Wednesday, with many areas now accessible only by air.

Seven out of ten states in grossly underdeveloped South Sudan, the world's youngest country born just over two years ago following some five decades of on-off civil war, have been declared disaster zones by the government.

The UN Office for the Coordination of Humanitarian Affairs (OCHA) said that "severe flooding" due to intense seasonal rains had left 156,000 people in need of assistance.

"So far humanitarian partners have reached nearly 100,000 flood-affected people with aid across the country," OCHA said in a report.

"However, they face access challenges as roads have become impassible, with many flooded areas only accessible by air."

Flooding happens every year, but problems began earlier this month after rains were more intense than expected.

The majority of those affected are in Northern Bahr el-Ghazal state, bordering Sudan, where over 45,000 are needing support.

In places, the White Nile river that runs the length of the country has burst its banks.

Minister for Cabinet Affairs Martin Elia Lomuro told state media that the situation was made "even worse since wild animals and human beings are sharing the little space left by the flood".

In Warrap state, in the north of the country, a crocodile has eaten one person forced from their home by the rising water, state media added.

South Sudan is oil-rich but remains one of the world's poorest countries, where even the most basic infrastructure, such as roads, electricity and water distribution networks, is lacking.

War-torn Jonglei in the east, a vast region riven by conflict between rival tribes and where rebels were accused of massacring scores of people at the weekend, has some 28,000 people affected.

Tit-for-tat cattle raids and ethnic killings are common in Jonglei, awash with guns left over from the last round of civil war, from 1983-2005.

The main rainy season in South Sudan usually runs from around June to the end of October, but flooding can affect areas up until December.