"The High Commissioner Navi Pillay is expressing extreme alarm at the dramatic deterioration of the situation in Iraq," Rupert Colville, her spokesman, told reporters in Geneva.
The rights chief was especially concerned by verified reports of "summary executions and extrajudicial killings and the massive displacement of an additional half a million people" by militants from the Islamic State of Iraq and the Levant (ISIL), he added.
In the past week, the fighters from the Islamist group have overrun a succession of major towns and cities and were today closing in on Baghdad.
Pillay "will be warning parties to the conflict that they are obliged under international law to treat members of the armed forces who have laid down their arms or are hors de combat humanely," he said.
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"Murder of all kinds, mutilation, cruel treatment and torture constitute war crimes," he added.
The UN had received disturbing reports after the capture of Iraq's second city Mosul, including the suicide of four women who had reportedly either been raped or forced to marry ISIL soldiers.
A court employee and 12 people believed to have been serving with Iraqi security services or police had been executed in the central Mosul neighbourhood of Dawasa, he added.
Militants have also carried out several kidnappings in Mosul, including the seizure of dozens of Turks from a consulate and 16 Georgians working for an Iraqi communications company.