GENEVA (NYTIMES) - Islamic State in Iraq and Syria (ISIS) militants have summarily killed scores of civilians in the Iraqi city of Mosul in recent days, sometimes using children as executioners, and have also used chemical agents against Iraqi and Kurdish troops, UN officials have said.

Video posted by the militants on Wednesday (Nov 9) showed four children, who appear to be 10 to 14 years old, shooting four civilians accused of disloyalty at a location near the Tigris River, said Ms Ravina Shamdasani, a spokeswoman for the UN human rights office in Geneva, on Friday (Nov 11).

The video release identified one of the children as Russian, another as coming from Uzbekistan and two as Iraqis.

UN investigators had not identified the time of the killings, but believed they were recent, citing the surge in executions by ISIS courts and fighters in and around Mosul in recent weeks and the brutal training the militants have forced on children in the parts of Iraq and Syria they control.

"They are showing they are still in business," Ms Shamdasani said of the ISIS.

In one massacre, militants were said to have summarily shot 40 civilians in Mosul, dressing them in orange clothes adorned with words, marked in red, labelling them "traitors and agents of the ISF", Ms Shamdasani said, using the abbreviation for Iraqi Security Forces.

Afterward, the militants strung up the bodies of their victims from electricity poles around the city - a practice the ISIS long used to strike fear into those who live in the group's strongholds.

The next day, ISIS fighters shot 20 civilians at a military base in the north of the city and also strung up their bodies with signs carrying statements like "used cellphones to leak information to the ISF", she said.

The battle for Mosul, with tens of thousands of security forces bearing down on Iraq's second-largest city, is now almost a month old.

Over the past week, the largest numbers of civilians so far have fled the fighting, with close to 48,000 people displaced as of Friday, according to the World Health Organisation and United Nations.

But that is nowhere near the total that officials worry could be in danger once the fighting moves to the most populated areas across the Tigris on the west side of Mosul, which is still believed to be home to at least 1 million people.

Reports from inside the city indicate that the ISIS has set up elaborate defences on the banks of the Tigris, including artillery pieces.