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, BBC News reports.

At least 204 are believed to have been shot dead by militants during clashes with Iraqi security forces in the Shifa district last Thursday, June 1 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 U.S.-led multinational coalition.

They managed to take full control of the eastern half of Mosul in January and started an assault on the west the following month.

Fewer than 1,000 militants are now besieged in IS-controlled parts of the Old City and several adjoining northern districts, along with some 100,000 civilians.

The UN Human Rights office said it had documented IS "use of civilians as human shields and its slaughter of those attempting to flee" since the start of the operation, but that recent reports indicated "a significant escalation in such killings".