The UN says one of its helicopters has been shot down in South Sudan, killing all four crew members.
UN deputy spokesman Eduardo del Buey said the helicopter from the peacekeeping mission in the country was on a reconnaissance mission when it was shot down.
The mission, known as UNMISS, said the helicopter was not carrying any passengers.
"In subsequent communications between the mission and the South Sudanese Armed Forces, the SPLA told the mission that it has shot down the helicopter in the Likuangole area in Jonglei state," Mr del Buey said. The SPLA is South Sudan's army.
A UN official aware of the incident, who was not authorised to speak publicly, said the Russian-built Mi-8 helicopter was painted white - the traditional colour for UN aircraft - and had shared its flight plans with South Sudan's military.
Kieran Dwyer, spokesman for the UN peacekeeping department, said: "It's extremely concerning that a UN helicopter appears to have been shot down by the host country's army, and the investigation has to get to the bottom of it."
Mr del Buey said the UN mission is carrying out an investigation.
Earlier, SPLA spokesman Philip Aguer told AP that "the helicopter crashed somewhere between the SPLA base and the Yauyau base in the Pibor area".
He said he did not know what caused the crash. "I think the government of South Sudan and UNMISS need to investigate this jointly," Mr Aguer said.
Mr del Buey said "initial reports indicate that the UN helicopter crashed and burned" after being shot down.
The UN mission immediately launched a search mission which "confirmed the death of all four crew members". They were not immediately identified.
South Sudan became the world's newest country in July 2011, the culmination of a six-year peace process which began with the signing of a peace agreement with Sudan in 2005 that ended more than two decades of civil war.
On the eve of its independence, the UN Security Council unanimously approved a new peacekeeping force for South Sudan to provide military and police support to help maintain and consolidate peace and security.
UNMISS currently includes over 6,500 troops and military liaison officers and about 550 international police.
A Sudanese official later denied responsibility - blaming rebel fighters in the region for the incident.
"The SPLA (South Sudan's army) did not shoot down the helicopter. Rebels of Yau Yau shot it down," army spokesman Kella Kueth told Reuters, referring to rebels led by David Yau Yau, who have been fighting the army in Jonglei state.