"Reports suggest the number of people killed in recent days may run into the hundreds," Navi Pillay, UN High Commissioner for Human Rights, said in a statement on Friday.
"I am especially concerned about the risk to vulnerable groups, minorities, women and children," Pillay added. She said that the Sunni militant group Islamic State in Iraq and Syria (ISIS) had a "well-documented record of committing grave international crimes in Syria."
The human rights chief also said she was deeply disturbed by reports that ISIS "fighters, including prisoners they had released from jails in Mosul and provided with arms, have been actively seeking out - and in some cases killing - soldiers, police and others, including civilians."
There had also been reports of "summary executions" of Iraqi army soldiers and civilians, according to Pillay.
The International Organization for Migration (IOM) has estimated that the fall of Mosul has displaced around half a million people. "Insecurity is spreading across the whole of Iraq and we foresee a protracted humanitarian crisis," said Mandie Alexander, IOM's emergency coordinator in Baghdad on Friday.
US, Germany concerned
Meanwhile, US Secretary of State John Kerry urged Iraqi Prime Minister Nuri al-Maliki "and all Iraqi leaders to do more to put sectarian differences aside." This reiterates long-standing Western complaints that the Shiite prime minister has done very little to heal sectarian rifts, leaving many of Iraq's minority Sunnis cut out of power since Saddam Hussein's demise.
During a meeting with his Moroccan counterpart Salaheddine Mezouar in Berlin, German Foreign Minister Frank-Walter Steinmeier said that halting the insurgency of the Islamic State of Iraq and Syria (ISIS) will require "an Iraqi government that can act authoritatively in its own country."
Militants from the Islamic State in Iraq and Syria (ISIS) overran the northern Iraqi city of Mosul earlier this week and have since thrust southwards toward Baghdad, the seat of the Shiite Muslim-led central government.
The developments signal what could be Iraq's greatest instability since the United States' 2011 troop withdrawal. On Thursday, Maliki's Baghdad government appeared paralyzed when parliamentarians denied him a quorum and power to declare an emergency.
ng/jr (dpa, Reuters, epd)