UNITED NATIONS – The United Nations said South Sudan’s armed forces shot down a U.N. helicopter on Friday killing all four crew members on board — but South Sudan’s military spokesman blamed rebel fighters.U.N. deputy spokesman Eduardo del Buey said the helicopter from the U.N. peacekeeping mission in South Sudan 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,” del Buey said. The SPLA is South Sudan’s army.Pibor County in Jonglei State, where the helicopter was shot down, has been the scene of recent clashes between rebel militia fighters led by David Yauyau and South Sudanese forces. A former member of the South Sudanese Army, Yauyau launched his rebellion after failing to win a parliamentary seat in the Sudanese general elections in April 2010. South Sudan accuses Sudan of arming Yauyau.South Sudan military spokesman Kella Kueth denied that his military shot down the U.N. helicopter and told AP: "It was the forces of Yauyau. Yauyau’s forces are working with Khartoum for the downfall of South Sudan.”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 didn’t know what caused the crash."I think the government of South Sudan and UNMISS need to investigate this jointly,” Aguer said.South Sudan Information Minister Barnaba Marial Benjamin, asked about the U.N. statement on SPLA responsibility, said: "That is New York, but UNMISS and us here on the ground are very clear that the cause of the crash is not known and needs to be investigated.”A U.N. official aware of the incident, who was not authorized to speak publicly, said UNMISS reported that SPLA fighters shot down the Russian-built Mi-8 helicopter. It was painted white — the traditional color for U.N. aircraft — and the U.N. had shared its flight plans with South Sudan’s military, the official said.Kieran Dwyer, spokesman for the U.N. peacekeeping department, told AP: "It’s extremely concerning that a U.N. helicopter appears to have been shot down by the host country’s army, and the investigation has to get to the bottom of it.”Del Buey said the U.N. mission is carrying out an investigation.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 U.N. 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.