South Sudan's warring parties are blocking food and medicines from reaching thousands of civilians who have fled more than three months of raging conflict, top UN aid officials have warned.

Around 50 trucks carrying some 2,000 tonnes of urgent aid supplies are currently held up across the war-torn nation, with officials from both government and rebel sides delaying aid by demanding paperwork or erecting multiple roadblocks, said John Ging, operations chief for the UN Office for the Coordination of Humanitarian Affairs(OCHA) on Friday.

"There are too many checkpoints, there are too many delays," Ging told reporters in the South Sudanese capital Juba, according to the AFP news agency.

"Assistance is urgently needed, people are hungry. People are in need of urgent medical life-saving assistance, children need nutritional supplements. They cannot wait."

Violence erupted in South Sudan on December 15 between forces loyal to President Salva Kiir and fighters loosely allied to former vice president Riek Machar.

A ceasefire between government and rebels inked in January is in tatters with fighting ongoing, and peace talks failed to resume as scheduled this week as the two sides squabble over who can attend negotiations.

"We have food in the country here and we need to get it to them," Ging said, appealing to both sides to allow aid through.

Race against time

"We are in a life-saving business, there is no reason to delay," he told reporters, warning the UN were "racing against time" to deliver aid by road before torrential seasonal rains made mud tracks impassable.

Earlier this week UN peacekeeping chief Herve Ladsous accused South Sudan of waging a campaign of harassment against peacekeepers, although government spokesman Ateny Wek Ateny has insisted the government will do all it can to work with the UN.

Ging was part of a team of emergency chiefs from eight UN agencies assessing the growing humanitarian crisis in the country.

Thousands have been killed nearly one million people forced from their homes since the fighting erupted.

Huge warehouses of food aid have also been looted.

"We cannot raise money internationally for humanitarian action if every time humanitarian action is robbed...it is not acceptable," Ging added.