Scores of suspected ISIS fighters are being held in a cramped jail in Mosul amid claims Iraqi troops committed human rights abuses retaking the city.

Pictures show men huddled together on the floor in 45C heat in a prison with no electricity or ventilation to the south of the war-ravaged city, which was recently free from the terror group's control.

The images, showing young and old suspected jihadists, emerged as an Iraqi lieutenant revealed the thirst for revenge among army ranks and insisted ISIS militants 'are not human beings.

He also revealed how he interrogated one suspect before 'sending him to hell' and insisted ISIS jihadists who brought terror to the country deserved to die.

Scores of suspected ISIS fighters are being held in a cramped jail in Mosul amid claims Iraqi troops committed human rights abuses retaking the city

Pictures show men huddled together on the floor in stifling conditions in a make-shift prison to the south of the war-ravaged city, recently liberated from the terror group

The Iraqi officer who oversees the make-shift jail facility said it currently holds some 370 prisoners.

He says authorities were overwhelmed with detainees as Iraqi forces cleared the last neighborhoods of the city earlier this month at the end of a grueling nine-month campaign.

'Prisoners are infected with diseases, lots of health and skin problems, because they're not exposed to the sun,' he said. 'The majority can't walk. Their legs are swollen because they can't move.' He said a provincial health team checks on the prisoners 'occasionally.'

More than 1,150 detainees have passed through the prison over the past three months, with 540 sent to Baghdad for further investigation, the officer said.

Another 2,800 prisoners are being held in the Qayara air base south of Mosul, and hundreds more in a few smaller facilities. The officer spoke on condition of anonymity because he was not authorised to brief reporters.

Prisoners who were discreetly interviewed by the AP insisted they were innocent. They spoke on condition of anonymity, fearing reprisals.

'You won't find 10 real (ISIS members) among these guys. And all of them have spent more than six months here,' one prisoner said out of earshot of the guards. 'Since I got here eight months ago, I've only seen the sun once.'

He said he was a civil servant and had traveled between Baghdad and Mosul on several previous occasions before being detained.

The Iraqi officer who oversees the make-shift jail (pictured) said it currently holds some 370 prisoners

The images, showing young and old suspected jihadists, emerged as an Iraqi lieutenant revealed the thirst for revenge among army ranks and insisted ISIS militants 'are not human beings

'They said my name was in their database. I haven't seen any court or judge. I don't even know what I'm accused of. A lot of names are the same,' he said.

He said two prisoners had died in the packed holding cell. Some prisoners 'have pus coming out of their wounds. Once they go to the hospital, they come back with amputated legs or arms.'

'We really want to die,' another prisoner said. 'None of us have received any visitors, relatives, family members. They don't even know where we are.'

The country's prime minister Haider al-Abadi has acknowledged Iraqi forces committed human rights violations, but insisted that these were 'individual acts' for which the perpetrators would be punished.

They said my name was in their database. I haven't seen any court or judge - I don't even know what I'm accused of

ISIS militants were notorious for atrocities, both against civilians and Iraqi security forces, often hunting down anyone connected with the police or military after they overran territory.

And for some Iraqi soldiers, the fight against ISIS in Mosul has been a slow, methodical quest for revenge. For three years, one lieutenant has hunted for two ISIS militants from his village who he believes killed his father.

Along the way, he has shot to death detained militants after interrogating them, he acknowledges unapologetically.

And if he catches either of the men he is searching for, the lieutenant vows he will inflict on him 'a slow death' and hang his body from a post in the village after forcing him to reveal where his father's body is buried.

That sort of thirst for vengeance in the wake of military victories is fueling extrajudicial killings of suspected ISIS members at the hands of Iraqi security forces in and around Mosul.

Videos that emerged last week showed troops in Mosul taking captured ISIS suspects and throwing them one by one off a high wall next to the Tigris River, then shooting their bodies below.

The Iraqi officer who oversees the make-shift jail facility said it currently holds some 370 prisoners

More than 1,150 detainees have passed through the prison over the past three months, with 540 sent to Baghdad for further investigation, the officer said

Speaking to The Associated Press, four Iraqi officers from three different branches of the military and security forces openly admitted that their troops killed unarmed and captured ISIS suspects, and they defended the practice.

They, like the lieutenant, spoke on condition of anonymity because they acknowledged such practices were against international law, but all those interviewed by AP said they believed the fight against ISIS should be exempt from such rules of war because militant rule in Iraq was so cruel.

However, the killings risk tipping Iraq back into the cycles of violence that have plagued the country for over a decade, according to Belkis Wille, Iraq researcher with Human Rights Watch.

ISIS was able to attract recruits in the past because of people's anger over abuses, including arbitrary detentions, torture and extrajudicial killings, she said.

If abuses continue, 'all you're going to see is (that) young Sunni Arab men are going to want to join whatever the next extremist group looks like,' she said. Despite the military's vows not to tolerate it, she said no soldier or commander has been held accountable for any killings.

The bloodshed reflects the deeply personal nature of the fight against ISIS.

When the militants overran Mosul and large parts of northern and western Iraq in 2014, they specifically targeted members of the military and security forces and their families for brutal atrocities.

Video has emerged of a man in an Iraqi Army uniform moments before he shoots an unarmed man in Mosul, Iraq. Vengeance is reportedly fueling a string of extrajudicial killings of suspected ISIS militants

The video shows a man kneeling before he is gunned down by a man in an Iraqi military outfit

Near Tirkrit, ISIS massacred some 1,700 captured military recruits and buried them in mass graves that have been uncovered since.

Hundreds of policemen and soldiers in Mosul are believed to have been killed after the takeover. Militants made no attempt to hide atrocities.

Defense Ministry's spokesman, Brigadier General Tahseen Ibrahim, said that authorities 'have not registered any incident of revenge killing, whether carried out by security forces or residents. The situation is under full control and we will not allow such incidents to happen because this issue is very sensitive and leads to violent reactions.'

When you're facing a man who has killed your friends, your family, yes, sometimes the men get rough

But a senior Iraqi officer said his troops regularly killed men who were said to be ISIS among civilians fleeing the city at screening centers in and around Mosul.

He spoke on condition of anonymity because of the possibility it could prompt legal repercussions.

'When an entire group of civilians tells us, "This man is Daesh," yes, we shoot him,' he said, using the Arabic acronym for ISIS.

'When you're facing a man who has killed your friends, your family, yes, sometimes the men get rough,' he added. 'But for us, this is personal.'

The lieutenant said the two men who killed his father were well known in his hometown, a small village south of Mosul. He agreed to share his story with the AP because he wanted to show how personal the fight is for Iraqi troops.

Two of his colleagues confirmed his version of events. The AP is not revealing the names of the men he is pursuing because there is no way to confirm independently they belonged to ISIS.

Pictured left: A man is beaten by troops in what appear to be Iraqi army uniforms. Right: He panics and pleads as the soldiers prepare to drag him off towards a wall before throwing him to his death

The lieutenant said his father was an officer in the security forces who fought al-Qaeda, the predecessor to ISIS, in 2007, at the height of Iraq's sectarian violence.

After ISIS seized the village in 2014, the tribes that were once kicked out for al-Qaeda ties moved back in, and ISIS installed them in security and administrative positions.

According to the lieutenant, two men grabbed the lieutenant's father outside his home. The two were among those previously expelled for al-Qaeda ties, he said.

I'm not selfish with my revenge - what I'm doing is for all Iraqis

The lieutenant was away, and his neighbors told him his father had been killed and who did it. He said he was told the men boasted about it in public. ISIS fighters also killed the lieutenant's uncle and more than a dozen other friends and relatives.

The lieutenant keeps an old picture of the two men on his phone. He said a handful of other troops know about his hunt and have helped him interrogate and kill ISIS suspects.

As Iraqi forces advanced toward the lieutenant's village last year in the lead-up to Mosul, he began interrogating captured ISIS suspects.

'Most of them I just asked questions,' he said, 'but for those who I knew had blood on their hands, I killed them on the spot.'

He said he has killed more than 40 militants, whether in combat or in interrogations on the sidelines of the battle. He acknowledged most were not directly responsible for his relatives' deaths.

Pictured left: The man is dragged up a short path to the top of the precipice. Right: The group of soldiers forcing the ISIS fighter to the edge can be seen gathering around him

Pictured left: Blood can be seen covering the man's shirt as the troops drag him to the cliff edge. Pictured right: The man - who it is claimed is an ISIS fighter - is brought to the precipice moments before he is executed

'I'm not selfish with my revenge, what I'm doing is for all Iraqis,' he said.

Early on in the Mosul operation, he said he learned that one of the two men was in Tal Afar, a town west of Mosul that remains in ISIS hands, or had fled to Syria.

In early July, as Iraqi forces pushed into Mosul's Old City, he received a tip on the location of the second man. He said a colleague, an intelligence officer, called and said he was holding an ISIS suspect from the lieutenant's home town.

'I told him don't do anything, keep him there. I'm on my way,' the lieutenant said.

The detainee was the uncle of the lieutenant's second target. The man was left alone with the lieutenant in a bare concrete room without a table or chair.

'I didn't torture him. I cut the plastic handcuffs from his wrists and gave him water,' the lieutenant said. The man was elderly, with a grey beard and hair.

'He begged me not to kill him as I questioned him,' he said, smiling. 'He could barely walk (he was so scared).'

Eventually, the man told the lieutenant that his second target was alive and in Mosul's Old City.

Pictured left: The lifeless body of another man can be seen on the ground, suggesting multiple executions. Pictured right: The man is thrown from the cliff edge and, moments later, shot with a rifle several times

'After I questioned him I sent him to hell,' the lieutenant said flatly. He said he shot the man with his side arm and left his body on the floor.

The first reports of revenge killings appeared within weeks of the launch of the Mosul operation last year and continued throughout. But the government and rights groups do not have an exact number.

In June, Human Rights Watch said at least 26 bodies of blindfolded and handcuffed men had been found dumped in government-held areas in and around Mosul. A month later, HRW said it had further reports of extrajudicial killings. Wille of Human Rights Watch said it was taking place 'basically everywhere that is touched by this conflict' and by every armed force involved in the fight.

The military says troops have orders to hand any captured ISIS over for interrogation ahead of future trial.

Prime Minister Haider al-Abadi on Tuesday acknowledged that rights violations took place during the Iraqi forces' battle in Mosul but described them as 'individual acts' by persons who were either 'ignorant' of the consequences or who had struck a deal with Daesh with the intent 'to defame us and the security forces.'

He pledged the government would punish the perpetrators.

The lieutenant dismissed the idea of going to the courts, saying they are corrupt and suspects could bribe their way to freedom.

'I know some people believe that this kind of killing is wrong, but Daesh, they are not human beings,' he said. 'I am the one who still has my humanity.'

When al-Abadi declared 'total victory' in Mosul last week, the lieutenant said he believed his target is still in one of the last ISIS pockets in the Old City.

'I hope I find him alive,' he said, 'because I want to make sure he dies a slow death, not quick. I want him to tell me where my father's body is buried, and then I want to take his body and hang it from a post in my village.'