The UN children's agency says at least 129 children were killed - with boys castrated and girls raped - during a government offensive last month in South Sudan.
UNICEF said the killings took place over three weeks in May in Unity state as the government battled rebels.
Speaking in New York on Wednesday, UNICEF Executive Director Anthony Lake said survivors reported that boys were castrated and left to bleed out, while girls were gang raped and killed.
South Sudanese military spokesman, Colonel Philip Aguer on Thursday questioned the credibility of the report, saying it is not in South Sudanese culture to commit such atrocities.
He called for full investigations, saying whoever is responsible must face justice.
South Sudan has been fighting rebels led by the former deputy president since December 2013.
