People in Mosul call it “the Biter” or “Clipper” – a metal instrument newly introduced by Isis officials to punish women whose clothes they claim do not completely conceal their body. A former school director, who fled from the city earlier this month, describes the tool as causing agonising pain by clipping off pieces of flesh.

Fatima, a 22-year-old house-wife who does not want to give her full name, said she had finally escaped from Mosul after several failed attempts because her children were starving and Isis had become more violent and sadistic compared with a year ago, especially towards women.

“The Biter has become a nightmare for us,” Fatima said after reaching safety in Mabrouka Camp for displaced people near Ras al-Ayn in Kurdish-controlled north-east Syria. “My sister was punished so harshly last month because she had forgotten her gloves and left them at home.”

Isis insists that women be fully veiled, wear loose or baggy trousers, socks and gloves, and be accompanied by a male relative whenever they step outside their homes.

Fatima said that a month after the use of this metal tool to punish her sister “the bruises and scars are still visible on her arm.” She quoted her sister as saying that “the biting punishment is more painful than labour pains.” Other witnesses describe the Biter as operating like an animal trap, or a metal jaw with teeth that cut into the flesh.

It is difficult and dangerous to escape from Mosul, which Isis has held since capturing it from the Iraqi army in June 2014. But people from the city, who have had themselves smuggled across the border to Syria and then to Kurdish-controlled territory known as Rojava in the past two months, all confirm that living conditions have deteriorated sharply. There are serious shortages of almost everything including food, fuel, water and electricity.

Isis was violent from the start of its rule 20 months ago, but public whippings and executions have become far more common in recent months. Mosul residents say that Saudi and Libyan volunteers, who have joined Isis, are the most likely to impose penalties for minor infringements of regulations in the self-declared caliphate.

It is as if Isis fighters and officials are compensating for setbacks in the war by showing that they still have power over the population under their control.

Ibraham, a 26-year-old pharmacist who left Mosul on 16 January, said that there is little food and only a limited supply of medicine left in the city. “My pharmacy became half empty,” he said. Pharmaceutical factories around Mosul have stopped production and there are fewer medicines being imported from Syria. Simple painkillers like Panadol that cost $1 (70p) for a bottle last year now cost $8, according to Ibrahim.

There is a shortage of food and what is available is very costly. The “caliphate” is increasingly cut off from supplies from Turkey and the rest of Syria. It also has less money to spend because of air attacks on its exports of crude oil, combined with the fall in the price of oil.

The Baghdad government continued to pay the salaries of public servants in Mosul even after Isis took over, but Ibrahim said that money stopped coming through nine months ago. “I have spent almost all my savings,” he said. “Last year, $500 a month was enough for a family to live on, but now even $1,000 is not enough because prices are twice or even five times what they used to be.”

Refugees speak of starvation spreading throughout the city under the impact of this economic siege. “For me, I could stand the bad treatment and lack of food, but when my toddler of 11 months began to starve it became impossible to stay,” said Fatima.

Baby milk has not been available for six months and other foodstuffs are prohibitively expensive. Rice costs $10 a kilo. Nor are these problems confined to Mosul. Farmers are leaving their fields because “there is no electricity to pump water so they cannot irrigate their crops”, according to Ghanem, 25, an unemployed plumber who is now in north-east Syria.

He insists that the main reason he fled Mosul was not the bad living conditions, but Isis “poking their noses into the details of people’s daily lives with their arbitrary fines and punishments”. He speaks of the increasingly harsh treatment of women, with the Biter being used as a punishment “on women deemed to have shown too much skin”.

Popular revulsion against Isis within the “caliphate” does not necessarily translate into resistance or mean that its rule is fatally undermined, however. There have been few anti-Isis armed attacks in Mosul and Isis uses its well-organised and merciless security arm to target real and imagined opponents. Where tribes have risen up against Isis in Iraq and Syria their members have been hunted down and slaughtered in their hundreds. Whatever the shortages affecting the ordinary population in Isis-held territory, officials and fighters will not go without food or fuel – though falling revenues does mean that their salaries have been cut in half.

The Caliphate is under heavy attack from its numerous though disunited enemies, the most important of which are the Syrian and Iraqi armies, the Iraq Kurdish Peshmerga and the Syrian Kurdish People’s Protection Units (YPG). These armies are not very large, but their fire power is greatly multiplied by the close support they receive from US and Russian air strikes. This makes it impossible for Isis to hold fixed and identifiable positions without suffering serious casualties.

But Isis can still act as a skilled and experienced guerrilla force, attacking vulnerable roads such as that linking Syrian government-held Homs and Aleppo, which the group cut this week.

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Even so, there are clear signs of growing corruption and disorganisation within Isis. The very fact that so many people have escaped from Mosul, despite strict rules against leaving, shows that Isis is less capable of enforcing its regulations than previously, for all the terror that it still inspires. The former school director, who does not want to give his name, says: “They threaten to kill us if we go outside Mosul”. Smugglers commonly charge between $400 and $500 to secretly transport someone to safety, though some of this may go straight to Isis which is desperate for money.

Ghanem said he was frightened at first as he left Mosul for Syria, but a smuggler reassured him saying: “Don’t worry. Money makes everything possible and they [Isis] will take their share.”

Isis was always a paranoid organisation, seeing traitors and spies everywhere, and this is growing worse. Anything can be grounds for suspicion: one woman, who eventually reached safety in Erbil, mentioned casually that her brother-in-law had been arrested and executed because he had once been a member of a police unit that specialised in protecting the oilfields.

Wisam, a 19-year-old student, had worked in a minor capacity as a photo editor in the local TV station and for news agencies, an activity he thought might put him at risk. “I spent more than a year working in the bazaar selling vegetables,” he said. “I could not work online because the internet is heavily monitored by Isis.”

Mosul is returning to a pre-modern era without electricity or drinking water, say its former inhabitants. During the first year of its Caliphate, Isis made great efforts to ensure that public services worked as well as, or better than, under the Iraqi government, but it appears to have abandoned the attempt.

“We only get drinking water once a week,” said Wisam. “Pipes are broken and need repair, but the administration in Mosul has become careless and confused over the past five months.”

The mains electricity supply has likewise almost stopped and people rely on private generators, either their own or those owned by local businessmen who sell the power. This can be too expensive for many families. Fatima said that “most areas of the city are dark and Mosul has become like a ghost town.”

Dependence on generators means reliance on locally produced fuel, which is of poor quality since US air strikes have destroyed the refineries in Syria that were controlled by Isis. The fuel cannot be used in cars and damages the motors in generators, which often stop working. Isis tried a coupon system to ration fuel but later abandoned it. Ghanem said that “we feel we are living in the Stone Age: no mobiles, no TV, no cars, even no lighting.”

The pressure of war on many fronts, combined with the tightening economic blockade, has undermined the Caliphate’s attempt to show Sunni Arabs that it is better able to administer a state than the Iraqi or Syrian governments. When its fighters captured Ramadi in May last year they got credit from local people for swiftly reopening the local hospital, something the Iraqi government had failed to do, by bringing in doctors from Syria.

They also brought in large generators to provide electricity. In much of eastern Syria, Isis’s draconian regulations were preferable to the criminality and insecurity which had flourished previously under other armed opposition movements.

The testimony of refugees is inevitably biased against those who forced them to abandon their homes and flee and, while the accounts of their suffering are undoubtedly true, they cannot speak for those who stayed behind. Isis still has fanatical supporters and there is no mass exodus of deserters from its ranks, even though they are being bombed by the two largest air forces in the world.

Isis was always infamous for relating to the rest of the world solely through violence and, as the tide turns against it on the battlefield, it is not surprising that this violence is becoming steadily even more extreme.