The group hurtled across western Iraq and eastern Syria over the summer in a sudden offensive that shocked the world. Isis remains a formidable force: it controls swaths of territory and continues to make progress in western Iraq. But its fighters have reached the limit of discontented Sunni Muslim areas that they can easily capture and US-led coalition air strikes partnered with offensives by local ground forces have begun to halt their progress.

Activists and fighters in parts of eastern Syria controlled by Isis said as military progress slows and focus shifts to governing the area, frustration has grown among militants who had been seen as the most disciplined and effective fighting force in the country's civil war.

Flagging morale, desertion and factionalism are starting to affect the Islamic State of Iraq and the Levant, known as Isis, testing the cohesion of the jihadi force as its military momentum slows.

The US military announced this week that air strikes had killed two senior Isis leaders — though there has been no confirmation of the claim by the group — and on Friday Kurdish peshmerga fighters broke the jihadis' five-month siege of Mount Sinjar in Iraq.

"Morale isn't falling — it's hit the ground," said an opposition activist from Isis-controlled areas of Syria's eastern Deir Ezzor province. "Local fighters are frustrated — they feel they're doing most of the work and the dying . . . foreign fighters who thought they were on an adventure are now exhausted."

An activist opposed to both the Syrian regime and Isis, and well known to the Financial Times, said he had verified 100 executions of foreign Isis fighters trying to flee the northern Syrian city of Raqqa, Isis's de facto capital. Like most people interviewed or described in this article, he asked for his name to be withheld for security reasons.

"After the fall of Mosul in June, Isis was presenting itself as unstoppable and it was selling a sense of adventure," a US official said. He added that the dynamics have changed since the US launched air strikes in August and helped break the momentum of the Isis advance, which has helped stem the flow of foreign recruits — though he warned that the change of mood "doesn't affect the hardcore people of Isis".

Analyst Torbjorn Soltvedt, of Verisk Maplecroft, a UK-based risk analysis group, said morale may be taking a hit as militants grapple with the shift from mobile army to governing force.

"Before they were seizing territory, forcing armies in Iraq and Syria to retreat," he said. "Now they're basically an occupying force trying to govern."

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After flocking to Syria and Iraq during Isis's heady days of quick victories, some foreigners may also be questioning the long, gruelling fight ahead.

Mr Solvedt said his organisation has had many reports of foreign fighters, including Britons, contacting family members and state authorities seeking ways to return home.

Isis members in Raqqa said the organisation has created a military police to crack down on fighters who fail to report for duty. According to activists, dozens of fighters' homes have been raided and many have been arrested. Militants told a local journalist that they must now carry a document identifying them as a fighter and showing whether they are assigned to a mission.

An opposition activist in close contact with Isis fighters in Raqqa showed the Financial Times a document listing new regulations restricting jihadis' behaviour. The paper, which could not be verified and which did not appear to have been issued in other Isis-held areas, warned that those who did not report to their offices within 48 hours of receiving the regulations would be punished.

"In Raqqa, they have arrested 400 members so far and printed IDs for the others," the activist said.

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The identification document for one fighter from the Gulf consisted of a printed form stating "name, location, section and mission assignment", with his details filled in by hand.



"The situation is not good," he grumbled, adding that fighters have become increasingly discontented with their leaders. He refused to give more details, saying only: "We aren't able to speak the truth and we are forced to do useless things."

Activists in Isis-held parts of Syria said many fighters in Raqqa were angry about being sent to Kobani, a small Kurdish town near the Syrian border with Turkey that has become a focal point for coalition strikes. The fighters argued that the town was not strategically important enough to justify the losses they were incurring. According to a December 7 report by the Syrian Observatory for Human Rights, a UK-based monitoring group with a network of activists across Syria, Isis lost about 1,400 fighters in 80 days of fighting. The US official said many Isis fighters have been killed in the town.

Foreign militants have often been the most active in major battles but opposition activists said as fighting intensifies, more demands are being made on local fighters who do not have deep-rooted loyalties to Isis.

"They pledged allegiance to Isis so they could keep fighting the [Assad] regime and not have to go against Isis," the Deir Ezzor activist said. "They feel they are the ones going to die in big numbers on the battlefield but they don't enjoy any of the foreigners' benefits — high salaries, a comfortable life, female slaves."

Another problem, locals said, may be a rise in tensions among ethnic groups. Many fighters apparently group themselves by ethnicity or nationality — a practice which undermines Isis's claim to be ridding Muslims of national borders.

A widely publicised example was a clash between Uzbek and Chechen fighters in Raqqa in early November over control of some villas near the captured Tabqa air base.

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"Just like the Uzbek and Chechen issues in Tabqa, we are having similar issues in Manbij with the Tunisians," said an activist in Syria's northern city of Manbij. "They won't let some of the highest level security members [of other nationalities] on to their bases."

Residents in Raqqa also said they have seen growing signs of discontent. One man recalled a speech at the Fardous mosque last Friday by a Tunisian cleric who often appears in Isis videos.

"He urged the brothers to put aside their disputes, and said all brothers should stay together as one hand," the man said. "Now I realise why the preacher was saying this . . . Something is wrong."

(A journalist in Isis-held territory contributed reporting to this article. The name has been withheld for safety reasons.)