Unimiss condemned what it called "the widespread sexual violence, killings including beheadings, burning alive of civilians, and attacks on humanitarians in Leer County".

"These are amongst the human rights violations documented during a surge in violence carried out by armed youth from Koch and Mayendit counties,” Nicholas Haysom, Special Representative of the Secretary-General for South Sudan and Head of Unmiss, said in a press release on Monday.

Some 40,000 people were reportedly forced to flee their homes.

Two survivors of the attacks were repeatedly raped after they came out of hiding to find food for their children, Mr Haysom said.

Another woman who had recently given birth recounted that she was raped and severely beaten for three days.

Last week President Salva Kiir formed a committee to investigate the causes of the violence which occurred between 17 February and 7 April.