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United Nations officials cataloged Boko Haram's latest horrors for a special session of its human-rights arm —including "mass graves," the murder of enslaved women and girls they call "wives," and the use of children as "cannon fodder."

Zeid Raad al-Hussein, the U.N. High Commissioner for Human Rights, did not specify if any of the girls reportedly killed were those abducted in April 2014 from the town of Chibok. U.S. officials could not confirm that the girls cited by the U.N. were from Chibok, and one noted that since the kidnapping that sparked the the "Bring Back Our Girls" campaign, Boko Haram has taken many more women and girls in northeast Nigeria.

Coalition forces of Nigeria, Chad, Cameroon and Niger have reportedly had some successes in recapturing territory previously held by Boko Haram — and that "has brought to light gruesome scenes of mass graves and further evident signs of slaughter by Boko Haram," Zeid said.

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Zeid said Boko Haram was turning children into "cannon fodder," and a U.S. official backed up the assertion, describing the youngsters as "human shields."

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Zeid provided the new details about the terror group's atrocities as Nigeria was preparing for the inauguration of a new president elected over the weekend. He told a rare special session of the UNHCR in Geneva that since 2009, when Boko Haram began resorting to boundless violence, an estimated 15,000 people have been killed — including "countless" children and women, some of whom were forced into terrorist activities.

U.N. refugee agency figures show 100,000 Nigerians have already fled to neighboring Niger. In recent months, U.N. officials said, the numbers have jumped to 66,000 in Cameroon and 18,000 to Chad.

“This despicable and wanton carnage, which constitutes a clear and urgent menace for development, peace and security, must be stopped,” Zeid said. “Boko Haram’s leaders must know that they will be held accountable in a court of law for these appalling violations of human rights.”

Zeid told the session that there is a need for "strong, coordinated and principled" responses from within and without the region.

While the coalition has taken back towns, there are U.S. officials who believe Boko Haram had already left some of them in a tactical retreat — not military defeat.

For instance, in Damasak, the group left after abducting more than 500 civilians, many of them primary school students, one U.S. official said.

Following Zaid's remarks, several African countries on hand for the Geneva session called on the international community to intensify their response against Boko Haram. The United States has been providing intelligence support to Nigeria and the coalition, with Nigerian officials describing the help as "air assets" — reportedly unarmed reconnaissance drones flown out of Niger.

Over the weekend, former strongman Muhammadu Buhari swept to power in the country's presidential elections. On Wednesday, he promised that "Boko Haram will soon know the strength of our collective will and commitment to rid this nation of terror and bring back peace."