U.N. peacekeepers sexually abused and exploited local women and girls in Liberia (search), a U.N. spokesman said Friday.

Stephane Dujarric said a preliminary investigation by the U.N. mission in Liberia indicated that some allegations against its personnel could be substantiated while others could not.

"The allegations range from the exchange of goods, money or services for sex to the sexual exploitation of minors. The peacekeeping department here in New York as well as the mission on the ground are taking appropriate follow-up action," he said.

A U.N. official speaking on condition of anonymity said the total number of allegations could eventually total 20. The official said four U.N. nations contributed to the Liberian mission but declined to name them.

The head of the mission in Liberia, Jacques Paul Klein (search), is to step down when his contract expires at the end of the month, a U.N. spokesman announced Thursday. His deputy, Abou Moussa, will temporarily take over.

The allegations in Liberia are just the latest to be leveled against U.N. peacekeepers (search) who have been accused of sexually abusing the very people they were sent to protect in missions from Bosnia and Kosovo to Cambodia, East Timor and Congo.

"The United Nations treats this issue with the utmost seriousness, and as we continue to clampdown on misconduct throughout all peacekeeping missions it is very likely that the number of these allegations will increase," Dujarric said.

Last month, a U.N. report on peacekeeper sex abuse said the world body's military arm was deeply flawed. The report, written by Jordan's U.N. ambassador, Prince Zeid Al Hussein, recommended withholding salaries of the guilty and requiring nations to pursue legal action against perpetrators.

Currently, U.N. troops and employees accused of wrongdoing are sent home to be dealt with by their own government but are often never punished.

U.N. Secretary-General Kofi Annan commissioned the report after more than 150 allegations of sexual exploitation of girls as young as 13 by U.N. peacekeepers in Congo surfaced.