Fighting along the Nile River in South Sudan has prevented humanitarian aid from reaching more than 60 000 malnourished children in the northeast of the country for almost a month, two United Nations agencies said on Thursday.

The UN's World Food Programme (WFP) and agency for children (UNICEF) said they expect nutrition supplies for Upper Nile State, which has some of the highest rates of malnutrition in the country, to run out by the end of May.

"Children are already the first to suffer during emergencies. If we can't get nutrition supplies through, we are likely to see escalating malnutrition in areas already at breaking point," Mary-Ellen McGroarty, WFP's representative in South Sudan, said in a joint WFP-UNICEF statement.

The Nile is a crucial transport artery in South Sudan because the impoverished country has few paved roads and a lot of challenging terrain, particularly during the rainy season when many roads become impassable.

The agencies did not say which fighting had disrupted the route of their aid barges, but government forces have been fighting an ethnic Nuer militia known as the White Army in areas near the Nile since March.

The battles led to the arrest of First Vice President Riek Machar and a spiralling political crisis, which the United Nations has warned could reignite the brutal civil war that ended in 2018.

In mid-April, barges carrying 1 000 metric tonnes of food and nutrition supplies bound for Upper Nile State were forced to return due to insecurity, WFP and UNICEF said.

The agencies decided against pre-positioning supplies in health centres and warehouses in insecure areas because they could have become targets for looting, they said.

"We have reluctantly taken the unprecedented step of holding back supplies for fear that they will not reach the children that so desperately need them, due to the ongoing fighting, looting and disruption of the river route," said Obia Achieng, UNICEF's representative.