UNITED NATIONS — The uproar over allegations of child sexual abuse by French soldiers in Central African Republic spread on Friday, as the United Nations top human rights official accused France of delays in investigating its troops, the United States pressed for an inquiry, and the United Nations faced new scrutiny over the accountability of peacekeepers who exploit civilians they are sent to protect.

In his first public remarks since the allegations surfaced that French troops had sexually abused several boys from December 2013 to May 2014, Zeid Ra’ad al-Hussein, the United Nations High Commissioner for Human Rights, sharply questioned why France had not investigated its own soldiers before United Nations staff members began their own investigation.

The United Nations collected testimonies from six boys, some of whom described in detail how they had been sexually abused; others said they witnessed it.

“How is it that nobody knew about these abuses between December and May?” said Mr. al-Hussein, a former United Nations peacekeeper in Bosnia who, in a previous role, had pressed the United Nations to improve how it handles sexual abuse allegations in its peacekeeping missions.