Mass graves have been discovered in South Sudan with evidence of horrific crimes against civilians including forced cannibalism, according to a report.

The shocking findings were revealed by the African Union Commission of Inquiry which had been tasked with investigating the civil war that has gripped the country for nearly two years.

The violence has left tens of thousands of people dead and the impoverished country split along ethnic lines – between the Dinka-led government and Neur opposition.

The report presented testimony that the atrocities may have been pre-planned by both government fighters and rebels.

'There are reasonable grounds to believe that acts of murder, rape and sexual violence, torture and other inhumane acts... have been committed by both sides to the conflict,' the report added.

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A South Sudanese government soldier inspects the body of a dead woman lying in the street in Bor at the height of the ethnic violence that rocked the country in January last year. A report by the African Union has found hundreds of Neur men were rounded up and forced to eat each other's flesh before being shot

Among the most shocking of many acts of 'extreme cruelty' identified in the report were claims of, 'draining human blood from people who had just been killed and forcing others from one ethnic community to drink the blood or eat burnt human flesh.'

More than two million people have fled their homes in a war marked by gang rapes and the use of child soldiers.

The government and rebels have repeatedly signed peace deals but the ceasefire has been broken numerous times.

Investigators, led by former Nigerian president Olusegun Obasanjo, found the conflict began with a skirmish between members of the presidential guard followed by the government-organised killings of ethnic Nuer civilians and soldiers.

But it disputed that there was a coup attempt by former vice president Riek Machar.

President Salva Kiir is a Dinka while Mr Machar is a Nuer.

The report found that hundreds of Nuer men had been rounded up by perpetrators who tortured their victims, including by forcing them to jump in fires or eat human flesh.

The report was scheduled for release months ago but its publication was delayed by the African Union's Peace and Security Council.

South Sudanese government soldiers wait to board trucks and pickups, to head to the front line to reinforce other government forces already fighting rebel forces near the town of Bor in January last year

It found that the killings were 'an organised military operation that could not have been successful without concerted efforts from various actors in the military and government circles'.

The report added: 'Roadblocks or checkpoints were established all around Juba and house to house searches were undertaken by security forces.

'During this operation male Nuers were targeted, identified, killed on the spot or gathered in one place and killed.'

Defence Minister Kuol Manyang Juuk described a shadowy 'group (that had) organised itself as Rescue The President.

It killed most people here (in Juba) - from (December) 15 to 18. It was even more powerful than organised forces.'

The group comprised some Dinka soldiers who had been mobilised following a 2012 border crisis with northern neighbour Sudan.

Some of these soldiers were moved south to Mr Kiir's private farm near Juba in 2013 and later participated in the killings, the report said, citing interviews with informants.

Amid the Juba killings, Mr Machar fled the capital and mobilised an insurgency which committed revenge attacks against the Dinka, sparking a cycle of violence which also included rape and the murder of people in churches and hospitals, according to the report.

Those revenge attacks occurred so quickly that they were also likely to have been co-ordinated, it added.