(CNN) ISIS murdered more than 200 civilians over three days as they tried to flee fighting last week in western Mosul, Iraq, the United Nation's human rights office said, citing "credible reports."
At least 231 civilians were killed starting May 26, the Office of the High Commissioner for Human Rights said, adding that the murder of civilians amounts to "war crimes."
In one reported case, ISIS slaughtered 163 civilians on June 1 and left their bodies in the street for days, the UN said. Days earlier, militants killed 27 people, including 14 women and five children; neighbors buried the bodies two days later.
Life, death and sorrow for Mosul civilians as war rages on
Ten-year-old Mariam Salim cries as she is treated at a makeshift clinic in western Mosul. She was trying to make a run for it with her family on June 4, 2017, when their house collapsed from an explosion. Most in her family are buried under the rubble of what was their home.
Salim siblings: From left to right, Mariam, Ina'am and Shamil Salim, got out of their house as it collapsed, killing a sister and burying a brother and both parents under the rubble. Iraqi security forces cannot get to the house because it is still under ISIS control.
An Iraqi man shouts that ISIS killed his son when he tried to escape from the Zinjili neighborhood in western Mosul to a government-controlled area. ISIS took control of Mosul -- the largest city in Iraq -- in June 2014. The operation to liberate it started in October 2016 -- and is still ongoing.
Salim Sa'ed smiles and hugs his neighbor after they both were able to escape the Zinjili neighborhood, one of the very few districts still under ISIS control in western Mosul, on June, 4, 2017.
After years living under ISIS, Iraqi civilians escape the fierce battle raging in the Zinjili neighborhood in western Mosul -- one of the very few districts still under ISIS control -- on June, 4, 2017. People passing by a CNN crew said since the operation to liberate Mosul from ISIS started in October 2016, food has been getting scarcer by the day. Some said they have been living on soup made only of water and flour.
Iraqi civilians escape the fierce battle raging in the Zinjili neighborhood in western Mosul on June 4, 2017. As these women passed by Iraqi security forces, the woman in the orange hijab screamed, "Where were you for the last three years?"
Dave Eubank from the "Free Burma Rangers" volunteer organization carries a little girl to safety after spotting her hiding under her dead mother's hijab for two days. The child was rescued in western Mosul as gunfire raked the area. Photo courtesy of Free Burma Rangers (www.freeburmarangers.org/)
"Shooting children as they try to run to safety with their families -- there are no words of condemnation strong enough for such despicable acts," UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein said in a statement.
"I call on the Iraqi authorities to ensure that those who are responsible for these horrors are held accountable and brought to justice in line with international human rights laws and standards," he said. "The victims of such terrible crimes must not be forgotten."
Watch: Journey to the front line
ISIS took control of Mosul, Iraq's second largest city, in 2014.
Iraqi forces have been attempting to retake the city since October. Security forces launched a large-scale military operation late last month to reclaim the remaining ISIS-held neighborhoods.
Read: US volunteers aid Mosul's wounded
The fighting has caused significant displacement. The Iraqi government said more than 742,000 people have left Mosul and surrounding areas since the offensive began.
The United Nations migration agency warned last month that the number of people fleeing western Mosul is soaring.