The UN Human Rights Office say it has received reports of 'mass killings' of civilians attempting to flee the Iraqi city of Mosul.

The organisation says it believes more than 231 civilians have been killed since May 26th, amid efforts by security forces to retake the western part of the city from Islamic State.

In once incident on June 1st alone, ISIL militants are reported to have shot and killed 163 civilians - including women and children - who were trying to flee clashes between ISIL and Iraqi security forces.

The bodies are said to have been left on the streets for several days, while an "undetermined number" of civilians also went missing.

An incident on May 26th is reported to have seen 27 people - including 14 women and five children - killed, while another on June 3rd is believed to have left 41 civilians dead.

UN High Commissioner for Human Rights Zeid Ra’ad Al Hussein said: “Shooting children as they try to run to safety with their families – there are no words of condemnation strong enough for such despicable acts.

“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. The victims of such terrible crimes must not be forgotten," he added.

The UN notes that murdering or attacking civilians constitute war crimes.

The agency's Human's Rights Office is also seeking further information on reports of civilians killed by air strikes in the city - with one strike in an ISIL-held area of the city reported to have left 50 to 80 civilians dead.

Mosul - which is around 350km from Baghdad - has been held by ISIL since 2014.

Iraqi and Kurdish forces have been battling to retake the city since last year. While the eastern part of the city is said to have been liberated, ISIL currently retain control of west Mosul.