The Office of the United Nations High Commissioner for Human Rights on Tuesday condemned the killings of civilians in Nigeria by Boko Haram militants.
A spokeswoman told reporters in Geneva, Switzerland, that the deliberate targeting of civilians by the Islamic extremists was "clearly prohibited by international law".
According to accounts from eyewitnesses, days of relentless violence in Nigeria have left hundreds of civilians dead, mostly women, children and the elderly.
The killings in and around Baga, a town in Borno state that lies in the northeastern corner of Nigeria, near the border with Chad, unfolded over several days after Boko Haram fighters seized a key military base there on 3 January.
UN chief Ban Ki-moon, as well as the United States and other countries, have condemned the Baga bloodshed, which has highlighted the increasingly brazen tactics of an insurgent movement in Nigeria's northeast, as well as the inability of Nigerian forces to respond effectively.
Boko Haram is also suspected of using a 10-year-old girl to detonate a bomb at a market in Maiduguri last Saturday, killing at least 10 people and seriously injuring others. The bomb exploded after explosives were found under the girl's clothing during a search, according to witness accounts.
The militants have also been implicated in deadly bombings in Potiskum in northern Yobe state, which is adjacent to Borno.
At the Geneva briefing, the UN spokeswoman said the use of a child to detonate a bomb was "not only morally repugnant but constitutes an egregious form of child exploitation under international law".