The UN has received reports that 231 Iraqi civilians have been killed by so-called Islamic State while attempting to flee Mosul over the past two weeks.
At least 204 are believed to have been shot dead by militants during clashes with Iraqi security forces in the Shifa district last Thursday and Saturday.
The UN said it had noted a “significant escalation” in such killings.
There are also reports of between 50 and 80 civilians being killed in an air strike on the Zanjili area on May 31.
Pro-government forces launched an offensive to retake Mosul in October with the support of a US-led multinational coalition.