Tribal raids killed 900 South Sudanese in Dec-Feb: UN - Reuters

South Sudan, which split away from Sudan a year ago, has been struggling to stamp its authority on an undeveloped country the size of France awash with weapons.

In one of the bloodiest episodes since independence, an estimated 7,000 heavily armed youths from the Lou Nuer tribe attacked villages belonging to the rival Murle in eastern Jonglei state at the end of last year, stealing tens of thousands of cattle and abducting women and children.

The onslaught forced tens of thousands of people to flee their homes, killed 612 people and triggered a wave of revenge attacks in which another 276 died, the United Nations said.

The report, by the U.N. Mission in South Sudan's (UNMISS) human rights division, put the death toll lower than initial estimates from local authorities which had said 2,000 to 3,000 people may have died.

It criticized South Sudan's army, the SPLA, for failing to adequately protect civilians despite early warnings of the attacks, and said UNMISS - which has about 5,780 troops, military observers and police in the country - had not been able to do so because of troop and equipment shortages.

In a statement, U.N. High Commissioner for Human Rights Navi Pillay said it was vital "the perpetrators and instigators on all sides are held to account."

As well as internal conflicts, South Sudan has suffered violent border confrontations with Sudan since it gained independence, six years after a peace deal that ended decades of civil war between north and south.

The two countries have clashed repeatedly along their ill-defined border, notably in April when fighting in the Heglig oil region killed hundreds, according to tallies on both sides

Conflict and corruption have left the new nation with some of the worst health and education statistics on the planet. Only a quarter of all adults are literate while the country has just about 100 km (60 miles) of paved roads.

The causes of inter-communal violence in Jonglei state range from arms proliferation and insecurity to marginalization and lack of development, the report said.

Jonglei consists of primarily pastoral communities whose lives revolve around cattle and cattle migration.

Cattle raiding linked to competition for scarce resources has been one of the main triggers of conflict between ethnic groups, which has cost an increasing number of lives, particularly since 2009, the report said.

(Editing by Alexander Dziadosz and Robin Pomeroy)