In the Syrian city of Raqqa, the group's self-styled capital, water and electricity are available for no more than three or four hours a day, garbage piles up uncollected and the city's poor scavenge for scraps on streets crowded with sellers hawking anything they can find to sell, residents say.
Videos filmed in secret by activists show desperate women and children clamouring for handouts of food, while photographs posted on the internet portray foreign jihadists eating lavish spreads, a disparity that is starting to stir resentment.
Much of the assistance that is being provided comes from Western aid agencies, who discreetly continue to help areas of Syria under Isis control.
The United States funds healthcare clinics and provides blankets, plastic sheeting and other items to enable the neediest citizens to weather the winter, a US official said.
The government workers who help sustain what is left of the crumbling infrastructure, in Syrian as well as Iraqi cities, continue to be paid by the Syrian Government, travelling each month to collect their pay in government-controlled areas.
There are also signs of falling morale among some of the fighters whose expectations of quick and easy victories have been squashed by US-led airstrikes.
There is no indication that the hardships are likely to lead to rebellion, at least not soon. Fear of draconian punishments and the absence of alternatives deter citizens from complaining too loudly, the residents said in interviews while they were on visits to neighbouring Turkey or over the internet.
But the deterioration is undermining Isis' self-proclaimed identity as a state.
For most citizens, the main interaction with Isis is with its ubiquitous police and security agencies, including the notorious Hesbah, which patrols the streets in quest of those transgressing the group's harsh interpretation of Islamic law.
Those rules continue to be rigidly enforced. Shopkeepers shut their stores five times a day for prayer. Smokers have quit for fear of the obligatory three-day jail sentence for a first offence - and a month for a second. Public executions for theft, blasphemy and dissent are on the rise.
A new punishment, for homosexuality, in which the accused is thrown off a high building, has been implemented twice in recent weeks.
Crime has plunged, and for many residents the order is a welcome alternative to the lawlessness that prevailed when more moderate Syrian rebels were in charge.
Syrians who lived for decades under the regime of President Bashar al-Assad are accustomed to obeying orders, and many have adapted to the new rules, said a government employee in the former tax department who collects his salary from the government, even though he is no longer working.
"Daesh are not as cruel as the regime was," he said, using an Arabic name for the jihadists. With Isis in charge, "if you don't do anything wrong - according to their standards, not ours - they will not bother you".
- Washington Post