Geneva (AFP) - South Sudan has encouraged fighters to rape women in place of wages while children have been burnt alive, the UN said Friday, calling the young nation one of the world's most "horrendous" human rights situations.

Grotesque rights violations could amount to war crimes, said a report on the world's youngest country from the United Nations human rights office.

The UN findings coincided with an Amnesty International report saying government forces deliberately suffocated to death more than 60 men and boys by stuffing them into a baking hot shipping container.

After gaining independence from Sudan in 2011, South Sudan erupted into civil war in December 2013, setting off a cycle of retaliatory killings that have split the poverty-stricken, landlocked country along ethnic lines.

The UN said it had evidence that fighters from pro-government militia which fight alongside the Sudan People's Liberation Army (SPLA) are compensated under an agreement of 'do what you can and take what you can.'"

"Most of the youth therefore also raided cattle, stole personal property, raped and abducted women and girls as a form of payment," the report said.

It also found that civilians suspected of supporting the opposition, including children, had being burnt alive and hanged from trees and cut to pieces.

"This is one of the most horrendous human rights situations in the world," UN rights chief Zeid Ra'ad Al Hussein said in a statement.

Both the government and rebel sides have been accused of perpetrating ethnic massacres, recruiting and killing children and carrying out widespread rape, torture and forced displacement of populations to purge their opponents from areas.

- Screaming in distress -

Amnesty, referring to an October incident in the central town of Leer, said it interviewed 23 eyewitnesses who saw men and boys forced into a container with their hands tied or saw the bodies later dragged away and dumped.

The London-based rights group blamed the atrocity, which happened in a Catholic church compound in the northern battleground state of Unity, on government soldiers.

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"Witnesses described hearing the detainees crying and screaming in distress and banging on the walls of the shipping container," the report said.

The incident was first reported last month by the Joint Monitoring and Evaluation Commission (JMEC), a regional ceasefire body pushing peace efforts.

The JMEC report said that those found alive were then killed, and that the only survivor was an eight-year old boy.

- War crimes -

The UN report found that most civilian casualties in South Sudan appeared not to be the result of combat operations, but of "deliberate attacks on civilians".

Condemning the government's "scorched earth policy", the UN said satellite images showed that towns and villages had been systematically destroyed.

Over a period of only five months last year, from April to September, the UN recorded more than 1,300 reported rapes in Unity, just one of South Sudan's 10 states.

One women told investigators she was stripped naked and raped by five government soldiers in front of her children on the roadside and then raped by more men in the bushes, only to return to find her children missing.

Another was tied to a tree after her husband was killed and forced to watch her 15-year-old daughter being raped by 10 soldiers, the report said.

Since the beginning of the conflict, the UN has received 702 reports of children affected by sexual violence, including gang-rape victims as young as nine.

The scale of sexual violence in South Sudan was "particularly shocking", the UN said.

"Given the breadth and depth of the allegations, their gravity, consistency and recurrence and the similarities in their modus operandi ... there are reasonable grounds to believe the violations may amount to war crimes," the rights office said.

It urged the rapid creation of a "hybrid court", as called for in an August 2015 peace agreement, to try perpetrations of grave violations.

If that fails, it called on the UN Security Council to refer the situation to the International Criminal Court in The Hague.