UN details series of reported ISIL abuses and warns that the armed group is stockpiling chemicals in civilian areas.

ISIL fighters have reportedly shot and killed scores of civilians in Mosul in recent days, according to the UN, which has also confirmed the discovery of a mass grave in the nearby town of Hammam al-Alil, in which more than 100 bodies were found.

In a brief published on Friday detailing a series of ISIL executions and abuses, the UN’s human rights office said that 40 people were killed by the armed group on Tuesday for “treason and collaboration” with the Iraqi forces and their allies closing in on the city during a major military push.

Dressed in orange jumpsuits, the bodies of the victims were hung from electrical poles in several areas around Mosul, the UN said.

On Wednesday evening, ISIL reportedly shot to death a further 20 civilians in the Ghabat military base in northern Mosul, also on charges of leaking information.

“Their bodies were also hung at various intersections in Mosul, with notes stating: ‘Decision of execution’ and ‘used cell phones to leak information to the Iraqi Security Forces (ISF)’,” the UN said.

The rights office also said the mass grave in Hammam al-Alil, south of Mosul, was just one of several ISIL killing grounds.

The site was discovered on Monday and contained the bodies of at least 100 people, including former ISF officers and ISIL detainees, as well as people killed for initiating anti-ISIL attacks since the beginning of the Mosul operation four weeks ago.

IN PICTURES: Life after ISIL on the outskirts of Mosul

“I’ve been in Erbil since the beginning of this military operation to retake the city of Mosul and we have documented hundreds of executions by ISIL,” Belkis Wille, a spokeswoman for Human Rights Watch (HRW), told Al Jazeera from Erbil.

“We are not surprised, unfortunately, to see mass graves like this one [in Hammam al-Alil]; it definitely is not the only one.”

Chemical weapons, human shields

The human rights body said they had received reports of ISIL stockpiling large amounts of chemicals in civilian areas in order to be used as weapons.

At least four people died from inhaling fumes after ISIL shelled and set fire to the al-Mishrag sulfur gas factory in Mosul on October 23.

The UN also said it had gathered evidence that teenagers and young boys were being used by ISIL as suicide bombers during the offensive, while young girls and women were being sexually exploited by the armed group’s fighters.

“Since 27 October, ISIL has been relocating abducted women, including Yazidi women, into Mosul city and into Tel Afar town,” the human rights body said in its brief.

“Some of these women were reportedly ‘distributed’ to ISIL fighters while others have been told they will be used to accompany ISIL convoys.”

Atrocities on both sides

But in its brief, the UN human rights office also urged the Iraqi government to ensure that the rights of Mosul civilians are met amid accusations of atrocities committed by government forces.

It cited sporadic reports of retaliatory attacks, including allegations of revenge killings by civilians or by forces under the control of the Iraqi army.

The UN High Commissioner for Human Rights, Zeid Ra’ad Al Hussein, said that he welcomed a statement by the Iraqi Prime Minister Haider al-Abadi condemning such acts, but urged the government to act to prevent reprisals and revenge killings.

“Justice for the victims and survivors of human rights abuses and violations – irrespective of when, where or by whom such abuses and violations were committed – need to be impartial, transparent and effective,” Zeid said.

“The government of Iraq must act quickly to restore effective law enforcement in areas retaken from ISIL to ensure that captured fighters and their perceived supporters are dealt with according to the law.”

One video circulating on social media on Friday appeared to show a teenage boy being shot and run over by a tank used by what seemed to be Iraqi-backed forces.

“I think we need to exercise extreme caution with videos like this,” HRW’s Wille told Al Jazeera. “We know that ISIL previously produced fake videos showing Iraqi forces committing abuses.”

There is a possibility that this video had been faked by ISIL fighters to shift the public opinion against Iraqi forces, she said.

“There is no flag on the tank in the video. That is a bit inconsistent with what I’ve seen on Iraqi force tanks,” Wille said. “Also only one man in the video is wearing an Iraqi uniform, and there are no other armoured vehicles in the area.”

But, she said, if the video is genuine, Iraqi authorities should take swift action to stop these kinds of extrajudicial killings.

“Unfortunately in the battle to retake Fallujah, we’ve seen multiple instances of abuses perpetrated by pro-government forces against the civilian population.

“And there is an extreme concern that this may happen again in Mosul.”

READ MORE: Iraqi forces accused of killing civilians near Mosul

Separately, Amnesty International on Thursday accused Iraqi police of torturing and executing civilians in villages near Mosul.

The UK-based rights group said its researchers had gathered evidence that up to six people were “extrajudicially executed” last month in the al-Shura and al-Qayyarah sub-districts of Mosul over suspected ties to ISIL.

Later on Thursday, Iraq’s federal police issued a statement denying its forces had been involved in extrajudicial killings.

Battle for Mosul rages

Meanwhile, Iraqi special forces said they pushed deeper into Mosul on Friday despite heavy resistance from ISIL fighters using civilians as cover, and were holding half a dozen city neighbourhoods seized in the past 10 days.

The elite Counterterrorism Service troops broke through ISIL defence lines to enter the city early last week and have since been embroiled in brutal, close-quarters combat with waves of suicide bombers and snipers.

The number of civilians killed in Iraq last month has nearly doubled since September, according to recently released figures by the United Nations Assistance Mission for Iraq.

The latest statistics reported 1,120 killed, up from 609 in September; and 1,005 civilian injuries, an increase from 951 in September.