The OIOS draft said 480 allegations of sexual exploitation and abuse had been made between 2008 and 2013, of which one-third involved children. It said missions in Democratic Republic of Congo, Liberia, Haiti and South Sudan accounted for the largest numbers of accusations.

In 2014 it said 51 allegations of sexual exploitation and abuse had been made against UN peacekeepers. States providing troops to UN missions have the primary responsibility to investigate allegations against their soldiers and police.

"Despite continuing reductions in reported allegations, that are partly explained by underreporting, effectiveness of enforcement against sexual exploitation and abuse is hindered by a complex architecture, prolonged delays, unknown and varying outcomes, and severely deficient victim assistance," OIOS said.

The draft included a response by the UN Departments of Peacekeeping Operations and Field Support. They regretted that OIOS did not evaluate prevention efforts and only focused on enforcement and remedial assistance efforts.

"This would have provided a much more complete view of the measures taken by the departments to address sexual exploitation and abuse," they said.

They did not dispute that underreporting was a concern, but noted there had been a significant increase in deployment of peacekeepers over the past decade and a large decrease in sexual exploitation and abuse allegations.

"This picture also supports an analysis that strengthened efforts ... are having a positive impact," they said.