The UN plays a key role in getting food to the thousands of South Sudanese who have fled their homes
Rebels in South Sudan have denied an allegation that they shot down a UN helicopter near the oil hub of Bentiu.
Three crew died and one survived when the Mi-8 cargo helicopter crashed.
South Sudan’s military spokesman, Col Philip Aguer, accused rebel forces of shooting it down.
Bentiu is one of the towns worst affected by fighting triggered in December when two factions of the ruling party fell out, but a ceasefire agreement is currently in place.
Thousands of people have died in conflict that started as a political dispute between President Salva Kiir and his former deputy Riek Machar but escalated into ethnic violence.
On Monday, the warring sides signed a deal reaffirming commitment to a ceasefire and were given 45 days to set up a transitional government.
The rebels, who are loyal to Mr Machar, said the allegations were “unfounded and baseless”.
Col Aguer had accused fighters under rebel commander Peter Gadet of shooting down the helicopter.
But rebel military spokesman Brig Gen Lul Ruai Koang denied this and said that the rebels would fully co-operate with the UN investigation into the crash.
Bentiu, the capital of oil-rich Unity state, has changed hands several times in the conflict.
The UN mission in South Sudan plays a vital role in getting food to the 1.8 million people who have fled their homes since fighting broke out.
Many have sought shelter in UN bases around the country, including Bentiu.
Up to four million people are at risk of food shortages because of the crisis, aid agencies say.