The UN has accused South Sudan's army of raping and then burning girls alive inside their homes during its recent campaign, a report by its mission in the country said.
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.
"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.
"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.
The military spokesman for the South Sudanese army, Philip Aguer Panyang, told Al Jazeera that the accusations made in the report needed further verification, and questioned accusations South Sudanese troops had obstructed UN investigators.
"Our role as an army is to facilitate humanitarian deliveries and access for civilian protection," Panyang said.
"If the UN has been denied access, they have the right to present those claims to the SPLA command."
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 as forces loyal to Salva Kiir tried to put down an uprising led by his former deputy, Riek Machar.
Peace talks between the factions collapsed in March this year, and clashes have since escalated.
Kiir said that he will not be forced into a premature peace deal and rejected the UN threat of sanctions against his country.
Source: Al Jazeera