A fighter uses a loudspeaker to address pedestrians in the ISIS stronghold of Raqqa.

Militants belonging to the Islamic State of Iraq and Syria (ISIS) have executed 100 members for trying to flee their base in the northern Syrian city of Raqaa, the Financial Times reported on Saturday.

An activist, who was identified by the newspaper as opposed to the Syrian regime and ISIS, confirmed to the FT the execution of the 100 foreign fighters attempting to flee Raqqa, the group’s de facto capital.

After being on the run for months, “frustration” began sweeping among the Islamist militants, the daily said.

Fighters sense a halt to ISIS’ military progress and are witnessing mounting casualties among their ranks, the report added.

“Morale isn’t falling – it’s hit the ground,” an opposition activist from ISIS-controlled areas of in Deir al-Zor told the newspaper.

Morale among the fighters was affected by the group’s shift toward governing areas it controls and because of U.S.-led air strikes halting their advances in Syria and Iraq.

“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,” the opposition activist told the daily.

Other activists and fighters in Raqqa told the paper that ISIS had created “military police to crack down on fighters who fail to report for duty.”

The activists said the ISIS “police” had raided the homes of several ISIS fighters and arrested many of them.

They also said ISIS militants reportedly need to carry identification documents to indicate if they are fighters and if they are allocated for a certain mission.

An opposition activist in touch with ISIS fighters reportedly showed the newspaper a document that lists “regulations restricting jihadis’ behavior.”

The paper, which the Financial Times said it could not verify, “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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