The United Nations voiced alarm Friday after recent mass kidnappings in Nigeria of hundreds of children and others, calling for investigations to ensure the perpetrators are brought to justice.

Criminal gangs often carry out mass kidnappings in northwest Nigeria, targeting schools, villages and highways where they can quickly snatch large numbers of people for ransom.

Nearly 600 people, including hundreds of school children, have reportedly been abducted in the past week alone, the UN rights office said.

"I am appalled by the recurrent mass abductions of men, women and children in northern Nigeria," UN High Commissioner for Human Rights Volker Turk said in a statement.

"Children have been abducted from schools and women taken while searching for firewood," he said.

"Such horrors must not become normalised."

In all, at least 564 people have reportedly been abducted since March 7, the UN rights office said.

Turk noted that Nigerian authorities had said they were working to rescue the children.

"I urge them to also ensure prompt, thorough and impartial investigations into the abductions, and to bring those responsible to justice," he said.

"The armed actors perpetrating these attacks must be identified and brought to account".

That, he said, was the necessary "first step towards reining in the impunity that feeds these attacks and abductions".