The Daily Telegraph of London reported on Tuesday that UN peacekeepers and civilian staff were raping and abusing children as young as 12 in southern Sudan.
The newspaper, in a story posted on its website, said it had gathered accounts from more than 20 young victims in the town of Juba of UN civilian and peacekeeping staff forcing them to have sex.
The UN Peacekeeping Department in New York declined to comment.
The report appeared on the first day of work for UN leader Ban Ki-moon of South Korea, who this week became the world body's eighth secretary-general, succeeding Kofi Annan of Ghana.
There are more than 11,000 UN peacekeepers and police from some 70 countries in southern Sudan, enforcing a January 2005 peace agreement that ended a 21-year civil war.
The Telegraph said the first signs of sexual exploitation of local youths in southern Sudan emerged within months of the peacekeepers' arrival in March 2005.
The UN Children's Fund UNICEF drafted an internal report detailing the problem, it said.
The newspaper said Sudan's government had gathered evidence including video footage of UN workers having sex with young girls.
But the United Nations has yet to publicly acknowledge there was a problem or even investigate, the newspaper said.
The United Nations, working with the African Union, is now pressing the Sudanese government to admit thousands more peacekeepers to its western Darfur region, where a separate civil war has raged for three years.
The government has been resisting letting the reinforcements into Darfur, calling it an attempt to recolonise the vast northern African nation.
Sexual abuse charges have surfaced for decades in UN peacekeeping missions and among civilian and other humanitarian staff operating in world hot spots.
But the United Nations began seriously pursuing offenders in the past two years after reports of widespread abuse in the Democratic Republic of Congo, where it has 17,000 troops.
Since January 2004, the United Nations has investigated abuse allegations against 319 military and civilian personnel in all its missions, the world body said in late November.
It has disciplined 179 soldiers, civilians and police since then but acknowledges minors and the poor are still exploited.