South Sudan's army shot down a UN helicopter Friday, killing all four Russian peacekeepers on board and bringing fierce United Nations condemnation on the world's newest state.
UN leader Ban Ki-moon said the helicopter was a "clearly marked" UN aircraft and demanded that those responsible be brought to account. The UN Security Council and U.S. government both deplored South Sudan's action.
Sudan People's Liberation Army (SPLA) troops fired anti-aircraft guns at the Russian MI-8 believing it was a rebel helicopter carrying weapons to anti-government forces, said a South Sudan military spokesman.
The helicopter was hit while on a "reconnaissance flight" over Jonglei state in eastern South Sudan, the deputy UN spokesman Eduardo del Buey said in New York.
Jonglei has been stricken by ethnic strife and rebellion since South Sudan became independent in July last year, with major help from the United Nations and western states. Relations have soured in recent months, however.
The UN Mission in South Sudan (UNMISS) at first only signalled that the helicopter had crashed in the Likuangole district of Jonglei.
"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," Mr. del Buey said.
"In subsequent communications between the mission and the South Sudanese armed forces, the SPLA told the mission that it had shot down the helicopter."
Ban "strongly condemns the shooting down today of a clearly marked UN helicopter by the Sudan People's Liberation Army," said a statement released by his office in which he expressed condolences to Russia over the crew deaths.
"In light also of previous incidents, the secretary-general demands that the government of South Sudan urgently puts in place reinforced measures within the SPLA to ensure that no such event may reoccur in the future."
It called for South Sudan to "immediately" undertake an investigation and bring those responsible to account.
The Security Council said it "strongly deplores" the "grave violation" of UN accords with South Sudan. The US State Department said it "deplores" the shooting down. Both called for a full investigation.
South Sudan military spokesman Philip Aguer said the UN chopper was hit by "friendly fire" and that an investigation had started.
He said the UN mission had been asked whether it had an aircraft in the Likuangole region and UNMISS had replied there were none. Mr. Aguer also alleged that the chopper was seen landing at a base belonging to rebel leader David Yau Yau.
"The plane took off while the SPLA forces were approaching the area. The SPLA artillery unit unfortunately fired on the plane," Mr. Aguer said. "Unfortunately, the plane was shot by friendly fire."
South Sudan is in the midst of its wet season and because of increased tensions in the region, the helicopter was looking for dry spaces to land peacekeepers if necessary, a UN official told AFP on condition of anonymity.
Likuangole, in the Pibor region, has been at the center of battles between rival tribes that have left thousands dead over the past year. Jonglei has also become a base for rebellion against the new government.
South Sudanese soldiers killed 14 people in the Pibor region this month in disputed circumstances.
Local officials said the victims were civilians. The army said the dead were armed followers of Yau Yau, an ethnic Murle.
Battles between the Murle, a minority group based in Pibor, and the state's Lou Nuer majority have brought mounting strife.
More than 600 people were massacred one year ago after an 8,000-strong mainly Lou Nuer mob went on the rampage in a district near Likuangole, according to a UN toll. Local officials say the figure was higher.
Last month, South Sudan's government expelled a UN human rights investigator looking into events in Jonglei. It accused the official of producing "unjustified" and "unethical" reports.
The United Nations strongly protested the expulsion to President Salva Kiir's government.
Human Rights Watch and Amnesty International have condemned a string of reported abuses -- including shootings, torture and rape -- as security forces crack down on those seen to support the rebels.
Jonglei was a frontline state in four decades of near continuous civil war between South Sudan and the Khartoum government up to 2005 that left at least two million dead.
The United Nations helped organize a self-determination referendum in January 2010 and the formal split came last year. Despite substantial oil reserves, the newest state is also one of the world's poorest.