The United Nations has said it will hold a conference in New York on Monday to address the issue of the sexual abuse of children by UN peacekeepers.
A teenager told her story to the BBC's Mike Williams
The issue was highlighted in a BBC report earlier this week, with claims of children being subjected to rape and prostitution in Haiti and Liberia.
The UN said it had not known about the case in Liberia and it could not substantiate the second case in Haiti.
But a UN spokesman said even one case of sexual abuse was one case too many.
The BBC inquiry was commissioned as part of Generation Next - a week of programmes focusing on people under 18.
"The sexual exploitation and abuse by UN personnel is unacceptable," UN spokesman Stephane Dujarric told reporters in response to questions about the BBC probe.
This Liberian teenager claims she became pregnant following abuse
"Over the past two years it has been clear we have redoubled our efforts in that regard to prevent these acts from happening, to discipline those who are responsible and to try to bring assistance to the victims," he said.
He emphasised the UN had a policy of zero tolerance towards sexual exploitation.
Commenting on specific allegations reported in the BBC investigation, Mr Dujarric said that one case in Haiti dated back to 2004.
He said the case had been "fully investigated" and "could not be sustained based on the evidence obtained".
Regarding another case involving a 15-year-old girl in Liberia, Mr Dujarric said the UN mission in Liberia, UNMIL, had not received any reports of "cases involving minors".
In May this year, another BBC investigation discovered systematic abuse in Liberia, involving food being given out to teenage refugees in return for sex.
The UN responded by heightening policing measures, appointing 500 monitors across the country, and introducing mandatory training of all personnel on appropriate conduct.
However, Mr Dujarric explained that about 80% of some 100,000 people who serve in peacekeeping operations cannot be disciplined by the UN.
"They belong to the various troop contributing countries, and we rely on those countries to discipline their personnel," he said. "
The UN's own figures show 316 peacekeeping personnel in all missions have been investigated, resulting in the summary dismissal of 18 civilians, repatriation of 17 members of Formed Police Units and 144 repatriations or rotations home on disciplinary grounds.
The UN says it has firm knowledge of only two concrete examples of sex offenders being sent to jail, although it believed there could be others it did not know about.