At least 129 children were killed, with boys castrated and girls raped, during a government offensive against rebels last month in South Sudan, according to the U.N. children's agency.

South Sudanese military spokesman, Col. Philip Aguer, on Thursday questioned the credibility of the report, saying it is not in South Sudanese culture to commit such atrocities and has called for a full investigation.

Speaking in New York 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.

Other children were thrown into burning buildings, he said, adding that the killings took place over three weeks in May in Unity state.

"In the name of humanity and common decency this violence against the innocent must stop," Lake said.

UNICEF had reported in May that dozens of children were targeted, citing witnesses who said those responsible were armed groups aligned with South Sudan's military.

South Sudan has been fighting rebels led by the former deputy president since December 2013, when a split among the armed forces in the capital, Juba, later escalated in bloody violence across the country.

Watchdog groups have accused both sides of carrying out serious rights abuses against civilians.