South Sudan army 'shoots down UN helicopter'
The UN says a helicopter from its peacekeeping mission in South Sudan has been shot down by the army, killing all four crew, believed to be Russians.
A spokesman said the aircraft was shot down while on a reconnaissance mission in eastern Jonglei state.
UN chief Ban Ki-moon strongly condemned the attack on the "clearly marked" helicopter.
However, South Sudan's minister of information told the BBC that the cause of the incident was not clear.
The UN has been helping those caught up in deadly clashes between rival communities in Jonglei state.
Deputy UN spokesman Eduardo del Buey said South Sudan's military had admitted it had hit the helicopter.
"Initial reports indicated the UN helicopter crashed and burned. The mission immediately launched a search and recovery mission. It has confirmed the death of all four crew members," the UN spokesman said.
"In subsequent communications between the mission and the South Sudanese armed forces, the SPLA [army] told the mission that it has shot down the helicopter in the Likuangole area in Jonglei state."
A spokesman for Mr Ban said: "The secretary-general strongly condemns the shooting down today of a clearly marked UN helicopter by the Sudan People's Liberation Army near Likuangole, in Jonglei State of South Sudan."
Mr Ban called on South Sudan's government to "immediately carry out an investigation and bring to account those responsible for this act", the spokesman said.
Earlier, South Sudan military spokesman Philip Aguer told AFP news agency that the helicopter had been hit by "friendly fire".
"The artillery unit unfortunately fired on the plane [believing] that this is an enemy plane because there was no prior information from the UN about this plane being in the area," he said.
"After 15 minutes of shooting at the plane, we heard that the UN had sent a plane," he added.'Rebel activity'
But South Sudan Minister of Information Barnaba Marial Benjamin told the BBC's Focus on Africa programme: "There is no proof it was shot down. It is being investigated.
"It was flying in an area where there is a lot of rebel activity."
The helicopter was identified both by UN and Russian sources as a Russian-built Mi 8 - a twin-turbine helicopter commonly used as a transport.
All four crew members were Russians, Moscow news agencies report, quoting Russian diplomats in South Sudan.
They also spoke of a fifth victim, who was not Russian but whose nationality was not given.
South Sudan accuses Sudan of backing rebel groups on its territory - charges denied by Khartoum.
Relations have been tense between the two countries since the South seceded in 2011.