File Photo: South Sudanese government soldiers sit in a pick-up truck at the military base in Malakal, northern South Sudan. AP
The bloodshed between February 17 and April 7 in Leer county in Unity state reportedly forced 40,000 people to flee their homes, with UN investigators recording 64 cases of sexual violence, the United Nations Mission in South Sudan (UNMISS) said.
"UNMISS strongly condemns the widespread sexual violence, killings including beheadings, burning alive of civilians, and attacks on humanitarians," it said in a statement.
In total "72 civilians were killed, at least 11 injured," the statement added.
Two women told a UN team they were repeatedly raped and gang-raped by armed youths when they came out of hiding to look for food for their children.
Another woman who had recently given birth suffered a similar fate and was beaten for three days.
"I am strongly appalled by these horrific attacks on civilians in Leer," said Nicholas Haysom, who heads UNMISS.
"We must all do everything we can to ensure that victims and survivors get the justice they deserve and receive the care and support they need."
Terrified villagers told AFP earlier this month about spending days hiding in swamps as armed men set fire to their huts and raided their livestock.
Some described horrific abuses including the rape of women and girls.
The violence has prompted fears of a return to conflict in the fragile young nation, which plunged into a civil war in 2013, barely two years after achieving its independence from Sudan.
The fighting between troops loyal to President Salva Kiir and his rival Vice President Riek Machar left nearly 400,000 dead before the two men agreed to lay down their guns in 2018.
But the country of 11 million has struggled to maintain a tenuous peace in the intervening years, grappling with lawlessness and explosions of interethnic violence.
Political bickering between Kiir and Machar has not helped, with both sides exchanging fire in recent months.
Although Kiir and Machar announced an end to the latest hostilities earlier this month and vowed to save the teetering peace pact, Unity state was wracked by fresh violence less than a week later.
Almost nine million people with children accounting for more than half will need aid to survive this year, the UN said this month.
"The Mission urges national and local authorities to take immediate measures to reduce tensions and prevent further escalations and retaliatory attacks," said Haysom, calling on perpetrators to be held accountable.
UNMISS was originally deployed for a year when the world's newest nation gained independence, but its mandate has been extended again and again as the country lurches from crisis to crisis.