The UN human rights chief has said there are "extremely distressing, credible reports" that Iraqis fleeing the fighting in Fallujah are facing extreme abuse and even death at the hands of Shia armed groups allied with the government troops.

Zeid Ra'ad al Hussein, citing witness testimonies, said on Tuesday that allegations of abuse included reports of multiple executions of men and boys, who were trying to escape the city held by fighters belonging to the Islamic State of Iraq and the Levant (ISIL) group.

"Eyewitnesses have described how armed groups operating in support of the Iraqi security forces are detaining the males for 'security screening'," Zeid said.

"[This] in some cases degenerates into physical violations and other forms of abuse, apparently in order to elicit forced confessions."

Rupert Colville, spokesman for the UN High Commissioner for Human Rights, told Al Jazeera, that they have received reports that at least 21 males had been summarily executed.

"We can't confirm the reports but they seem to be credible."

The Iraqi army is currently conducting a major military operation against fighters belonging to ISIL, also known as ISIS, outside Fallujah, close to the capital to Baghdad.

READ MORE: Will Fallujah put an end to ISIL in Iraq?

The UN human rights chief said that people escaping the town had suffered "two and a half years of living hell" under ISIL rule and faced not just enormous danger in escaping but also "double jeopardy in the form of serious human rights violations".

Iraqi authorities have a legitimate interest in vetting anyone fleeing ISIL-held areas to be sure they were not a security risk or suspected of involvement in atrocities, but the process had to be carried out by legal organs, the UN rights chief said.

"It is paramount that all individuals fleeing the violence around Fallujah must be assumed to be civilians without links to armed groups, unless there is clear and cogent evidence to the contrary," he said.

Zeid called on Iraq's government to immediately take concrete steps to ensure that it was putting into practice its pledges to protect civilians and bring people to justice for violating human rights.

Since January 2014, when Fallujah was captured by ISIL, at least 22,169 civilians have been killed and 43,435 wounded in Iraq, according to the Zeid's office. These figures do not include Anbar Province, where Fallujah is located.

In a seperate development, a car bomb in Iraqi city of Karbala killed at least 10 civilians and wounded 26 others on Tuesday, officials said.

The bomb went off in a commercial area in the city, which is some 90km south of Baghdad.

ISIL has claimed responsibility for the attack.

The Iraqi Army on May 23 began an offensive to drive out ISIL fighters from Fallujah [EPA]

Source: Al Jazeera and agencies