Islamic State fighters have killed scores of alleged spies or battlefield deserters in the past week and are dispatching boys as young as 12 to the frontlines with suicide belts as the group’s enemies advance on its last major strongholds.

Witnesses in Raqqa and Mosul – as well as the UN’s high commissioner for refugees – said Isis had also displaced tens of thousands of residents for use as human shields as it came under increasing military pressure.

The Iraqi army continues to make slow gains inside Mosul’s outer limits while, 260 miles (420km) to the west, a US-backed Kurdish force is edging towards Raqqa in Syria.



As each force advances, new details emerge of the savagery inflicted in the two cities. Nearly 48,000 refugees have fled Mosul, and many have described how the city’s Islamist occupiers are becoming ever more brutal.

Anyone suspected of collaboration with the encroaching Iraqi military has been killed, often in public squares. Several residents told the Guardian that in the past week, relatives and neighbours had been killed by Isis for reasons as trivial as carrying a mobile phone sim card.

According to the UN high commissioner for human rights. Zeid Ra’ad Al Hussein, Isis killed 40 alleged spies in Mosul on Tuesday and 20 more the following day.

On Monday, an underground prison was found in the Shura district, containing 961 men and boys who had been forced into cages as small as 1 metre by 0.5 metre. Many were emaciated and had been tortured.

Zeid said the unchecked brutality highlighted an urgent need for justice, truth and reconciliation. Without that, he said, revenge attacks and collective punishment would undermine efforts to reassemble the shattered communities of northern Iraq.



One man, Abu Lutfi, who arrived in a Kurdish-controlled area on Thursday, said he and his family had been among 500 people forced by Isis out of their homes and told to move to the west of Mosul. Lutfi said he suspected the terror group intended to use them as human shields against Shia militias. Instead, they made a perilous journey through Isis ranks and across lands conquered by Iraq’s national army.

“I swear they were getting even worse with the people,” he said. “At the beginning, they had asked sheikhs to swear allegiance to them, and many did. Now even those sheikhs are vulnerable. They are really paranoid about spies or collaborators.”

The wrath of the terror group has not spared its own members. Zeid said seven of its fighters were beheaded earlier this month for fleeing the battlefield, and Isis leaders had used loudspeakers to warn that the same fate would befall anyone else who tried to leave.

The high commissioner said Isis had moved sulphur from a giant pit near the town of Qayyarah that it ignited two weeks ago, to Shura, where a child reportedly died after inhaling sulphuric gas. Large quantities of sulphur had been stockpiled for use against advancing forces.



In Raqqa, Isis has been on a similar footing, stockpiling weapons, fortifying neighbourhoods and deploying child fighters, who have been indoctrinated through months of training, to areas where they suspect advancing Kurdish forces will attack the city.



A senior member of the organisation, with whom the Guardian has remained in contact over several years, said child fighters would be used in large numbers in the first and second lines of Raqqa’s defences.



“Many of these boys are sons of fighters who have been killed,” he said. “They have been programmed like robots. They fear nothing and are beyond being rescued.



“All the leaders know what is going on in Mosul. They are watching the news very closely and have been encouraged by the fact that the Iraqi security forces can’t go any further. They know how to fight them in the city, and they are confident that they can pin them down for a long time.



“In Raqqa it is different. It is much smaller, and harder to defend. There are not as many people to hide among. They know that Raqqa will be the last battle. But for now, they are busy preparing, while watching their televisions.”



Routes between Mosul and Raqqa remain open, despite the presence of large numbers of Shia militias west of Mosul, who have been tasked with blocking the escape of Isis members. For now, the extremists can move freely to Tel Afar, north-west of Mosul, which remains under Isis control. The towns of Billij and Baa’j, west of Tel Afar, remain firmly under the control of the group, as does the border town of Boukamel and the road inside Syria to Raqqa.



The Isis leader Abu Bakr al-Baghdadi is widely believed to have been in Tel Afar and Baa’j in recent weeks, where he enjoys the protection of up to four tribes.



“He moves in a small area and has done for a long time,” the Isis source said. “I’m not sure that he is playing a direct role in the battle.”



After expecting Mosul to fall within weeks of the campaign to free it, which began on 17 November, Iraqi and Kurdish officials are now privately talking about the war lasting up to three months. Iraqi officers had prepared for stiff resistance, but have been surprised by the military strength of the group and its commitment to fighting.



At the same time, command and control among Iraqi forces has slipped at crucial times, raising concerns that large-scale operations of the type needed to push further into the city are several weeks away from being launched.



The battle for Raqqa is almost certainly even further away, sources have told the Guardian. Turkish officials say they have been assured by Washington that the current US proxy Kurdish force will “isolate but not enter Raqqa”, and that no force will be told to take the city before the January inauguration of President-elect Donald Trump.

