The UN humanitarian agency said an estimated 1.03 million people have been affected by flooding across South Sudan since July.
The UN Office for the Coordination of Humanitarian Affairs (OCHA) said Jonglei region has been the worst affected with 404,000 people, followed by the Greater Pibor Administrative Area which has more than 125,000 people and Lakes area with 147,000 people.
At least 1,034,000 people have been affected by flooding, according to OCHA's latest humanitarian report released on Wednesday evening.
OCHA said unusually heavy rainfall from July to date has led to overflows of the rivers including Nile, Pibor, Sobat, Lol and other rivers as well as inland flooding, mainly in the eastern and central parts of South Sudan.
It also said flood response activities are considerably constrained by persistent heavy rains, flooded airstrips, insecurity and limited resources.
"There is an urgent need for additional logistic support with air assets and riverine transport to move cargo to priority locations," said the UN relief agency.
It said the flood-affected people's urgent needs include food and livelihood support, emergency shelter and non-food items, and health and protection services.