"The sharp rise in cases reported in the last three months is of concern," said Diane Corner, the deputy chief at the country's MINUSCA mission.
Corner told reporters via video conference from the capital Bangui that the alleged victims in the nine cases involving minors are of "a range of ages. The youngest is 11."
The 12,000-strong MINUSCA force, which took over from an African Union mission nearly a year ago, has been plagued by a series of allegations of rape and other misconduct by its troops.
Three new cases of rape allegedly committed by a MINUSCA contingent from the Democratic Republic of Congo came to light this week, and Congolese Justice Minister Alexis Thambwe Mwamba said Thursday he had ordered legal action.
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Of the 13 cases, one dating from December 2014 was closed after investigators concluded that the claims were unsubstantiated.
But the allegations in all 12 other cases remain under investigation, either by the country that contributed the accused soldiers or jointly with the United Nations.
Morocco and Burundi are investigating allegations of sexual abuse targeting their soldiers in MINUSCA that surfaced in June, UN officials said.
Under UN rules, it is up to member states to investigate and prosecute their soldiers who face accusations of misconduct while serving under the UN flag.
Ban told a special Security Council meeting last week that too many countries are slow in responding to accusations against their soldiers and in some cases do not respond at all.
The UN chief has denounced sexual abuse in UN peacekeeping as a "cancer in our system" and has vowed to name and shame countries that fail to take action.