"131 people have died," Hamadallah Adam Ali, the head of Sudan's civil defence authority, said.

The flooding has been most severe in regions where the Nile broke its banks after heavy rains, he added.

In a statement seen by Reuters on Wednesday, the United Nations warned of more rain.

"The flood situation remains critical. The early warning and emergency information centre predicts further heavy rainfall and the risk of flash floods," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.

"Access to affected areas for assessments and response continues to be difficult with roads becoming inundated and damaged and airstrips frequently not landable by fixed wing planes," the statement added.

Ali said the government has used tractors to carry aid across heavily flooded fields, as well as boats and helicopters to reach villages isolated by the deluge.

Justin Bagirishya, head of the south Sudan office of the World Food Programme (WFP), said some 16,000 people in remote villages in south Sudan have no access to humanitarian aid and need immediate assistance.

"They are cut off completely. There are no usable roads or airstrips," he told Reuters.

OCHA said 770 affected families from one town in south Sudan are now camped on the town's airstrip to escape the waters.

It said UN and other humanitarian agencies have used boats to deliver food and medical supplies and plastic sheeting to a number of hard-hit areas in south Sudan.