South Sudan often accuses Sudan of airdropping weapons to rebels in Jonglei.

Russia’s Itar-tass news agency quoted a source at the Russian embassy in South Sudan as saying the Mi-8 helicopter owned by Nizhnevartovskavia was working under a U.N. contract when it was downed.

Earlier this year, Russia said it would withdraw helicopters and personnel servicing the U.N. mission in South Sudan after voicing alarm at attacks on U.N. helicopters there.

In September, South Sudanese soldiers killed at least 10 troops when they shot and sank one of their own military riverboats in a remote region after mistaking it for an enemy craft, the army said.

South Sudan has been struggling since independence to build up state institutions in a country awash with weapons after decades of civil war with Khartoum ended with a 2005 peace agreement.

Human rights groups often accuse the SPLA, a loose group of former guerillas, of human rights violations and abuses. The army denies this.

Yau Yau, a former theology student, heads one of several militias fighting the government which accuses Sudan of supporting them. Sudan denies this.

A shortwave radio station with links to the Yau Yau rebellion earlier this year said the group was fighting the government in reaction to abuses committed during the disarmament program.