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, BBC News reported.

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.

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.

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.

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.