Geneva - The United Nations' top human rights official called on President Muhammadu Buhari on Friday to investigate reports of horrifying crimes by Boko Haram Islamist rebels and alleged abuses by the military.
Zeid Ra'ad Al-Hussein said he had seen allegations of mass executions, rape and amputations of children by Boko Haram - a day after two blasts killed more than 30 people.
There were also reports armed forces had mistreated people detained on suspicion of belonging to the group, he added.
"Civilians in northeast Nigeria have been living through horrifying acts of cruelty and violence by Boko Haram. These include wanton killings, summary executions, forced participation in military operations - including the use of children to detonate bombs, forced labour, forced marriage and sexual violence, including rape," Zeid said in a statement.
Buhari, who was sworn in a week ago, said on Wednesday that the army will take a bigger role in the effort to crush Boko Haram, by taking over from soldiers from Niger in occupying towns liberated from the Islamist militant group.
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Zeid, citing eyewitness testimony gathered by his office on atrocities committed by Boko Haram, said: "We have reports of children who were suspected of theft and had their hands amputated, of a man stoned to death on accusations of fornication, mass executions of captives whose hands and legs were bound and who were dumped into rivers and wells."
At least 1,000 people, "possibly many more," were brutally killed by Boko Haram in Mararaba Madagali in Adamawa State in late 2014, the statement said.
Other witnesses described how insurgents asked villagers in Kwajafa in Borno state in April to gather to hear them preach.
"When the villagers gathered, the insurgents opened fire. The U.N. Human Rights Office has also received a video recording of an execution, allegedly of a girl who refused to convert to Islam."
Zeid, referring to "extremely worrying reports" that had emerged about the conduct of Nigerian armed forces, said one man testified about his ordeal when he was mistaken for a Boko Haram member and detained by the military in Yola in Adamawa.
"The man said he spent five days without food or water, as detainees drank the urine of others to quench their thirst. He claimed that there was an average of five deaths per day in the facility," he said.
There was no immediate comment from the Nigerian government or armed services.