The landmark question-and-answer session before the UN's child rights watchdog in Geneva came as Pope Francis said Catholics should feel "shame", in an apparent reference to the scandals that have rocked the Church.
Francis has vowed zero tolerance of abuse, setting up a special committee to investigate sex crimes, enforce prevention and care for victims.
But top officials at the United Nations Committee on the Rights of the Child questioned the Vatican's resolve.
"The Holy See has not established any mechanism to investigate those accused of perpetrating sexual abuse, nor to prosecute them," charged committee member Sara Oviedo.
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Pressed for details of the Vatican's new committee, its UN ambassador, Monsignor Silvano Tomasi said the ground rules and membership were still being established.
Oviedo also quizzed the Vatican on specifics, demanding to know what it was doing in the case of Archbishop Jozef Wesolowski, a papal envoy from deeply Catholic Poland recalled from the Dominican Republic amid claims of abuse.
Monsignor Charles Scicluna, the Vatican's former top prosecutor, insisted the Church understood what it had to do.
"It's not words, it has to be commitment on the ground, on the level of the local churches," he told the panel.
But committee member Hiranthi Wijemanne said perpetrators were too often shielded by their hierarchy.