More than 40 South Sudanese officials may have committed crimes against humanity, the United Nations has said.

A report on Friday by investigators from the UN Commission on Human Rights in South Sudan said the officials - including military generals and state governors - may be responsible for the rape and murder of civilians and recruitment of child soldiers.

The officials were not named in the report.

"Children have been recruited by all sides in the conflict and forced to kill civilians; in many cases they have watched loved ones raped or killed," the report said.

"The scale of the hunger and destruction inflicted on the country by its political and military leaders defies description."

South Sudan plunged into civil war in 2013, two years after gaining independence from neighbouring Sudan, when conflict broke out between forces loyal to President Salva Kiir and others allied to his former deputy, Riek Machar.

Machar and Kiir signed a peace deal in August 2015 and formed a unity government the following April. It broke down three months later, and fighting has continued.

The conflict has resulted in the deaths of tens of thousands of people and left more than half of the country's 12 million population in need of humanitarian aid, according to the UN.

The report - based on interviews with hundreds of witnesses, satellite imagery and nearly 60,000 documents - will be used as evidence in future war crimes trials, due to take place under the terms of the 2015 peace deal.

Prospect of prosecutions

According to the agreement, South Sudan's government will create a hybrid court with the Africa Union to try people accused of atrocities.

Andrew Clapham, one of the report's commissioners, called for the court to be set up without further delay.

"Holding those in charge in South Sudan accountable for the intentional suffering they inflict on their own people is crucial to stemming this humanitarian catastrophe," Clapham said.

"The African Union should immediately move with South Sudan to establish the Hybrid Court."

The UN estimates nearly four million people have been displaced by the fighting in South Sudan, around half of whom are seeking refuge in neighbouring countries.

Individuals indicted for war crimes cannot hold or stand for office in South Sudan, according to the terms of the peace deal, Yasmin Sooka, chairperson of the UN Commission on Human Rights in South Sudan, said.

"Ultimately this is the only way to stop the rampant devastation of millions of human lives by South Sudan's leaders," she said.

Mawien Makol, South Sudan's spokesperson for foreign affairs, told Reuters news agency the government would prosecute anyone found guilty of committing crimes.

"This is a responsible government," she said.