A view of flooded agricultural land after Nile's water level rose in El Halfaya region of Khartoum, Sudan. (Photo: CFP)

The United Nations (UN) on Tuesday called for urgent action to protect the already vulnerable South Sudanese from the impacts of prolonged flooding and displacement as the wet season looms.

The UN High Commissioner for Refugees (UNHCR) warned that flooding and displacement are expected to worsen when the wet season begins in May.

"The approaching wet season threatens to swamp extremely remote communities where residents are already surrounded by floodwater," UNHCR said in a statement.

Record rainfall in the past three years and overflowing rivers have flooded thousands of hectares of farmland in eight states and prevented people from cultivating.

According to the UNHCR, 33 out of 79 counties are badly affected by the floodwaters, which have not significantly subsided since the last wet season.

It said the impacts are especially harsh in Jonglei, Unity and Upper Nile states, where thousands of people have been displaced, while others are marooned in dike-ringed compounds holding back floodwaters with mud, sticks and plastic sheeting.

"If we do not step up our support for the people of South Sudan, the climate crisis coupled with ongoing insecurity mean that resources will further shrink, leaving people with no means to survive," the UN refugee agency said, adding such climate events will worsen in the future, as extremes become the norm, not the exception.