Iraqi security forces secure a street as they clear al-Sajarya district on the eastern outskirts of Ramadi, the capital of Anbar province, on January 17, 2016, a few weeks after declaring victory against the Islamic State (IS) group. / AFP / MOADH AL-DULAIMI

After prayers, the traders of Bab al-Tob market in Iraq's second city, Mosul, formed a crowd around nine men lying on the street.

Among the prostrate men was an Iraqi soldier and an alleged Kurdish spy. A bulldozer approached and slowly ran over the prisoners, crushing them utterly.

More than a year after Islamic State (IS) conquered much of Anbar province and the plains of Nineveh the jihadists were, by last summer, beginning to lose ground.

In Mosul, the main IS stronghold in Iraq, the “caliphate” killings gathered pace.

They were, as the United Nations observed on Tuesday, a “warning” to potential deserters, spies and enemies.

The nine men murdered by bulldozers were among 18 802 people killed by IS over the last two years, according to a UN report released on Tuesday.

Between May and October, 3 855 people were killed by IS in Iraq alone, a 15 percent increase on the numbers from December to April.

The report also found that IS had by last summer enslaved an estimated 3 500 people in Iraq, mostly from the minority Yazidi community.

IS was continuing to murder innocent men, women and children, but was also increasingly killing suspects among its own fighters.

“It has started executing a lot of its own people, whether because they are fleeing from the front line or are suspected of other things,” a UN source said.

“And its abduction of children does make it look as if it is desperate to have enough fighters.”

The true number of IS fighters killed by their own group is difficult to determine. So too is the real number of civilian casualties which is likely to be far higher than UN estimates.

“Isil [IS] has also killed members of its own group for refusing to fight or acting against its interests...” said the UN report.

“The murders were frequently conducted in public spaces. [IS] displayed the bodies of its own members whom it had murdered as a deterrent to other members who might consider disobeying orders or otherwise acting against its interests.”

On 29 June, weeks after Iraqi security forces and Shia militias seized Tikrit from IS, a member of the jihadist group's “morality police”, al-Hisbah, was killed by a firing squad in Mosul.

That came a day after IS killed 32 of its own jihadists in Ramadi and Fallujah for “passing intelligence” to Iraqi forces. Ramadi was retaken by Iraqi forces in December.

Also in late June, a member of the Mosul administrative (Shura) committee, named as Abu Usman al-Hassan, who it was claimed was the representative in Mosul of the IS “caliph” Abu Bakr al-Baghdadi, was executed at al-Ghizlani military base in the city for “conspiring against the caliphate state”.

Then, in August, IS executed six of its members for fleeing a battle in Ramadi in Anbar province.

The UN said that by September, some 34 fighters from an insurgent group once classified as an IS ally had been executed after being found guilty of “apostasy and betrayal”.

On 16 September IS killed seven women from its al-Khansa Battalion for “disobeying orders”.

The victims were shot and their bodies were left at the entrance of the Battalion's headquarters in central Mosul as an example of what happens to those who disobey its orders.

As IS ranks have thinned through battlefield losses, desertions and the executions of suspected traitors, its recruitment has become increasingly desperate.

On 21 June, there were widespread reports that IS had abducted several hundred children aged between nine and 15 from various districts of Mosul.

The children, it was claimed, were “forced to undergo training” on Mosul's outskirts.

Children who refused to obey orders were flogged, tortured or, it was claimed by Kurdish authorities, raped.

The UN said on Tuesday that the number of abducted children had been between 800 and 900.

“The children were divided into two groups: those aged between five and 10 were placed in a religious education camp; and those aged between 10 and 15 were forced into military training,” the UN said.

“This report lays bare the enduring suffering of civilians in Iraq and starkly illustrates what Iraqi refugees are attempting to escape when they flee,” said the UN High Commissioner for Human Rights, Zeid Ra'ad al-Hussein.

“This is the horror they face in their homelands.”

Many of IS's killings last summer were strategic.

On 11 July, a former candidate from Qayyara for the Ninevah provincial council elections was shot in the head at al-Ghizlani.

A day later, another candidate for the council elections was similarly killed on Mosul's outskirts.

In August, three female candidates for parliamentary elections from the Mutahidoun Alliance, Iraq's largest Sunni bloc, were abducted from their homes in Mosul. All three women were shot in the head.

On 24 July, some 53 employees of the Independent High Electoral Commission in Iraq were abducted in Mosul.

The next morning, 28 of those abducted, including 11 women, were executed.

The jihadist group reserved particular cruelty for those it viewed as assisting the Iraqi government. In June, IS claimed to have murdered 16 men in three batches - by a rocket-propelled grenade fired at a car in which some men were placed, by placing others in a cage that was submerged into water, and by decapitating the remainder with explosives. The UN report said the men had been accused of co-operating with the Iraqi army.

The UN said IS had by last summer enslaved an estimated 3 500 people in Iraq, primarily women and children from the Yazidi community as the Islamists have faced setbacks.

As the children were taken to become IS fighters, Yazidi women were sold as sex slaves, fetching up to £1 400. Nineteen woman were killed in Mosul over two days at the beginning of August, after they refused to have sex with IS men.

It was claimed that Islamists had been “rewarded with the widows of executed men to motivate them to continue fighting”.

The UN report said: “On 21 June, in Mosul, it was reported that IS had announced a Koran memorisation competition in Mosul, stating that the first three winners would reportedly receive 'sex slaves' as prizes.”