A top UN rights official who had earlier admitted her negligence over allegations surrounding French soldiers' child sex abuse in Central African Republic (CAR) has stepped down.

The international body confirmed Flavia Pansieri has resigned from her position as deputy high commissioner for human rights.

The UN statement said she quit her post due to "health reasons" but did not offer more about her departure.

Several children, some as young as nine years, have alleged they were forced to have oral sex by French troops stationed in the conflict-ridden landlocked African nation. The allegations inflicted a blow on the UN operations in CAR subsequent to which UN Secretary General Ban Ki-moon ordered a thorough probe into the matter.

In a confidential note sent to UN authorities, dated 26 March and obtained by the Associated Press, Pansieri wrote: "I regret to say that in the context of those very hectic days, I failed to follow up on the CAR situation."

An internal review at the UN in July 2014 found that the French soldiers accused of sexually assaulting children were not serving under the UN command at that time. The abuse is thought to have taken place between December 2013 and June 2014.

After the spiralling allegations, the UN notified the French authorities who pledged to take severe action against the perpetrators. Nevertheless, no arrests have been announced as yet.

"If some soldiers have behaved badly, I will show no mercy," French President François Hollande said in April 2015 when the allegations came to light.

The French defence ministry said the government "was made aware at the end of July 2014 by the UN's high commissioner for human rights of accusations by children that they had been sexually abused by French soldier".