The body of a man blindfolded and with his hands bound is pictured on a roadside south of Mosul. (Reuters photo)

BAGHDAD, June 8 (Xinhua) -- The United Nations human rights office on Thursday said that "credible reports" showed that the Islamic State (IS) militants killed at least 231 civilians who tried to flee the battleground in the western side of Mosul.

A statement by the Office of the High Commissioner for Human Rights (OHCHR) said that all the reported killing took place in al-Shifaa neighborhood between May 26 and June 3, when the extremist militants opened fire on families who tried to flee the neighborhood. Many of the deaths were women and children.

"The bodies of those who died were reportedly left on the street for several days following the killings. In addition to those who died, an undetermined number of civilians went missing," the statement said.

The OHCHR also said that they are seeking further information about reports of civilian casualties caused by air strikes, including in the IS-held al-Zanjili neighborhood, where some 50 and 80 civilians were killed on May 31 by an air strike.

"Shooting children as they try to run to safety with their families ... there are no words of condemnation strong enough for such despicable acts," the statement quoted the UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein as saying.

The High Commissioner called on the Iraqi security forces and the anti-IS international coalition to ensure that all possible measures are taken to avoid casualties among civilians.

So far, the troops recaptured large parts of the al-Zanjili and al-Shifaa, but heavy back and forth street battles are still underway, as the extremist IS militants showed stiff resistance, while heavy clashes and bombings are underway at the edges of city center.

Earlier, UN Office for the Coordination of Humanitarian Affairs (OCHA), said the number of civilians remaining in the old city and surrounding neighborhoods cannot be known exactly, but the office estimated 200,000 may leave their homes in the coming days of battles.

"We don't know for sure how many civilians are still in IS-held districts, but as many as 200,000 additional people may try to leave in coming days," an OCHA report said on May 18.

According to the Iraqi military, over 90 percent of the city has been retaken from IS in the ongoing major offensive launched in October last year.

Mosul, 400 km north of Iraq's capital Baghdad, has been under IS control since June 2014, when government forces abandoned their weapons and fled, enabling IS militants to take control of parts of Iraq's northern and western regions.