"In the 100 days since the start of the conflict in South Sudan, over one million people have fled their homes," the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) said ina report late on Friday.
Over 800,000 are displaced inside South Sudan, while almost 255,000 have fled as refugees to neighbouring countries of Ethiopia, Kenya, Uganda and Sudan, the UN says.
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.
"Fighting between government and opposition forces has continued, especially in Jonglei, Unity and Upper Nile state, where towns and rural areas have been ravaged by the violence," the OCHA report added.
The conflict has caused a "serious deterioration in the food security situation" with some 3.7 million people at high risk, it read.
Peace talks in the Ethiopia capital have made little if any progress, with the two sides spending time squabbling in luxury hotels over who can attend negotiations.
Tens of thousands of civilians are sheltering inside UN peacekeeper bases fearing revenge attacks, crammed into tiny areas in increasingly squalid conditions due to heavy rains.
The UN estimates five million people are in need of aid, with vast swathes of the countryside increasingly difficult to reach by road due to seasonal heavy rains.
Huge warehouses of food aid stored for the rainy season before fighting broke out have been looted.
The UN World Food Program has therefore begun delivering food and medical supplies by costly air drops. In places without an effective runway the food sacks are simply dropped out of the back of giant cargo air planes.
UN children's agency chief in South Sudan Jonathan Veitch warned on Friday of "worrying signs of malnutrition and disease outbreaks" and that every effort had to made to "avert a humanitarian catastrophe."
Chris Nikoi, head of WFP in South Sudan, said he was "enormously concerned that things could get worse".