The United Nations human rights chief accused the Daesh terrorist organization Tuesday of murdering 163 civilians to prevent them from fleeing Iraq's western Mosul last week. "The brutality of Daesh and other terrorist groups seemingly knows no bounds," said Zeid Ra'ad Al Hussein.

"Yesterday, my staff reported to me that bodies of murdered Iraqi men, women and children still lay on the streets of the al-Shira neighborhood of western Mosul, after at least 163 people were shot and killed by Daesh to prevent them from fleeing," he said in his opening address to the UN Human Rights Council.

"My staff have also received reports of missing people from this neighborhood," he added, without providing further details.

His spokesman Rupert Colville told AFP that the killings were believed to have taken place on June 1.

Backed by the U.S.-led international coalition, Iraq last October launched a wide-scale military offensive to recapture Mosul and the surrounding areas, with various Iraqi military, police and paramilitary forces taking part in the operation. The city's eastern half was declared liberated in January, and the push for the city's western section, separated from the east by the Tigris River, began the following month.

Iraq's second-largest city, Mosul fell to Daesh in the summer of 2014 as the militants swept over much of the country's north and central areas. Weeks later the head of the terrorist group, Abu Bakr al-Baghdadi, announced the formation of a self-styled caliphate in Iraq and Syria from the pulpit of a Mosul mosque.

The Mosul offensive, now in its eighth month, has taken much longer than expected, with Iraqi government advances slowed by the need to avoid civilian casualties.

Zeid condemned "in the strongest terms the cowardly and sickening attacks perpetrated against innocent people by callous terrorists operating in many parts of the world."

Nearly 800,000 civilians have fled northern Iraq's Mosul city since the start of the military operation in February while an estimated 180,000 others remain in Daesh-controlled areas, according to U.N. figures.

"Humanitarian concerns remain high for the protection of an estimated 180,000 civilians still in Daesh-controlled areas in Mosul. "Since the start of the military operation in western Mosul on 19 February, nearly 600,000 civilians have fled western Mosul city."