By Tom Esslemont

LONDON, Sept 17 (Thomson Reuters Foundation) - The head of the United Nations stabilisation force in the Central African Republic, Parfait Onanga-Anyanga, has said there must be no mercy for peacekeepers who commit sexual abuse offences, adding that he expected more cases to come to light.

The 11,000-strong security force, known as MINUSCA, has been mired in a series of sex abuse scandals sparked by the revelation earlier this year that soldiers from France, Equatorial Guinea and Chad had assaulted several boys over a period of six months.

In his first interview since becoming the U.N. Secretary General's senior representative in CAR in August, Onanga-Anyanga called for an end to a "culture of impunity" and ordered countries sending the troops, six of them African nations, to enforce tougher penalties against perpetrators.

"Our commitment is absolutely strong ... They (the perpetrators) will be investigated, they will be punished with the harshest possible measures. This is not going to be business as usual," Onanga-Anyanga told the Thomson Reuters Foundation.

The Gabonese diplomat was appointed five weeks ago, after the scandals prompted the U.N. secretary general to sack his Senegalese predecessor, Babacar Gaye, saying "enough is enough".

Since Onanga-Anyanga's arrival in CAR the total number of sexual abuse cases has reached 17, including allegations against military, police and civilian MINUSCA staff.

"Human nature seems prone to errant behaviour ... even though all the measures are in place to prevent this," said Onanga-Anyanga, speaking from MINUSCA's office in the capital, Bangui.

U.N. peacekeepers, mainly from six African countries as well as Pakistan and Bangladesh, have been stationed in CAR since April 2014 with the aim of protecting vulnerable women and children affected by inter-communal and inter-religious violence that flared up the previous year, killing thousands.

The United Nations has not made public the nationalities of peacekeepers accused of sexually abusing women and children in CAR in subsequent scandals in June and August, but it has been revealed that the soldiers were from the Democratic Republic of Congo, whose military has its own history of sexual violence.

The veteran U.N. diplomat, who has previously served in Burundi, Vienna and New York, has called on the U.N. member governments to put alleged perpetrators on trial, but said the United Nations needed its own power to punish offenders.

"Fifteen years ago there were calls for U.N. investigators to be able to collect DNA evidence from crime scenes, to allow us to enter (those places) so that victims could have their say," he said.

"I am not giving up on this - the need for us to go into crime scenes," he added.

Onanga-Anyanga said the vulnerable nature of CAR's post-conflict society, where sexual abuse and prostitution are pervasive, made the scandals even more unacceptable.

"A man in U.N. uniform has a natural position of power, which he should not be using to take advantage of an already fragile society," he said.

Strict rules are already in place; fraternisation between MINUSCA troops and civilians is banned, and curfews are in place for military personnel (6 p.m.) and civilian staff (10 p.m.), but Onanga-Anyanga said these protocols would need to be strengthened.

Improved training regimes will be introduced to address "systemic" problems stemming in part from MINUSCA's inheritance of a contingent of African Union troops, who follow completely different standards and protocols, he said.

The MINUSCA chief called for greater "transparency" in his mission, including the protection of whistleblowers, which he said would likely lead to an increase in the number of reported cases. But he added that he did not want to distract from the progress he feels the mission is making in bringing security to CAR.

"The U.N. flag is flying, it is blue, and we will not allow it to be tarnished by the unacceptable actions of a few."

(Reporting by Tom Esslemont, editing by Tim Pearce.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, corruption and climate change. Visit www.trust.org)