The statement, published on Tuesday, warned the recent upsurge in fighting had been marked by a "new brutality and intensity".
"The scope and level of cruelty that has characterised the reports suggests a depth of antipathy that exceeds political differences," the UN said.
Members of the UN mission in Sudan (UNMISS) said they interviewed 115 victims and eyewitnesses in Unity state where South Sudanese forces were involved in fighting against opposition fighters in April.
The survivors allege that the Sudan People's Liberation Army (SPLA) killed civilians, destroyed villages and displaced 100,000 people, the statement added.
The UN said attempts to corroborate the reports were prevented by the SPLA, which denied its teams access to the areas concerned, Al Jazeera reported.
"UNMISS human rights officers visited two additional sites of alleged atrocities and conducted more interviews of eyewitnesses and victims. The information gathered from those visits and interviews provided further corroboration of the earlier accounts," the statement read.
UN officials said South Sudan's foreign ministry had been given early access to the report's findings, but had yet to comment.
"We call on the SPLA to fulfil this commitment and allow our human rights officers unfettered access to the sites of these reported violations," said Ellen Margrethe Loej, the head of UNMISS.
South Sudan attained independence in 2011 but the country has disintegrated into chaos. Thousands of people have been killed and almost two million displaced in a civil conflict that erupted in late 2013.