French President Francois Hollande has said there will be "no mercy" for any soldiers found to have abused children in the Central African Republic.
The authorities are probing claims that 16 French soldiers abused children at a camp for internally displaced people.
The Guardian newspaper reported that a leaked UN report documented alleged abuses of children as young as nine.
France sent an initial 1,600 troops to the country in December 2013 after violence flared following a coup.
Mr Hollande said there would be "sanctions in line with the amount of trust we put in the army" for any soldiers found guilty of abuse.
"I am proud of our armed forces, and so will show no mercy to those who have behaved badly, if that is the case," he went on.
The defence ministry has said the allegations, if proven, would be "an intolerable attack on military values".
The Guardian said that the alleged abuse took place between December 2013 and June 2014 at a centre for internally displaced people in the Central African Republic capital, Bangui.
Quoting an internal UN report, it says that in one case, a nine-year-old boy and his friend were forced to carry out a sex act by two French soldiers.
In some cases, it says, children were able to give good descriptions of the French soldiers alleged to be involved.
On Wednesday, the Guardian newspaper reported that a United Nations worker was suspended after leaking the report to French authorities.
A spokesman for the UN Secretary General said the leaking of an unredacted report constituted "a serious breach of protocol" and could endanger victims and witnesses.
France intervened in its former colony in December 2013, nine months after a rebel alliance, Seleka, captured the capital and ousted President Francois Bozize.
The country descended into ethnic and sectarian violence, with thousands of people fleeing their homes and the UN warning that there was a high risk of genocide.
The UN took over and expanded the African peacekeeping mission in September 2014.