The documents in the Isis file hinted at signs of rebellion within the ranks of its foreign fighters.

A Belgian militant had a medical note saying he had back pain and would not join the battle.

A fighter from France claimed he wanted to leave Iraq to carry out a suicide attack at home. Several requested transfers to Syria. Others just simply refused to fight.

The documents on 14 "problem" fighters from the Tariq Bin Ziyad battalion - made up largely of foreigners - were found by Iraqi forces after they took over an Isis (Islamic State) base in a neighbourhood of Mosul last month.

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At its peak, Isis drew thousands of recruits each month and controlled about a third of Iraq's territory, and the foreigners who poured in from dozens of countries have been characterised as the most die-hard fighters. But the group has steadily lost ground and appeal.

The militants are now besieged in the western half of Mosul, once the biggest city Isis controlled and the heart of its self-proclaimed caliphate. But the group's losses have triggered concerns in Europe that disillusioned fighters might find their way home.

"He doesn't want to fight, wants to return to France," said the notes on a 24-year-old listed as a French resident of Algerian descent. "Claims his will is a martyrdom operation in France. Claims sick but doesn't have a medical report."

He was one of five fighters in the file listed as having French residency, or as originally from France.

More citizens from France have joined Isis than from any other country in Europe since 2011, when Syria's popular uprising against President Bashar Assad turned violent and fuelled the rise of extremist groups.

The French Government reported a sharp decrease in the number of its citizens travelling to Syria and Iraq to join the group in the first half of 2016 but said that nearly 700 still remain there, including 275 women and 17 minors.

The forms in the file are marked with the year 2015 but appear to have been filled out later as they specify the dates that some of the militants joined, which stretch into 2016.

In addition to each militant's name, country of origin, country of residency, date of birth, blood type and weapons specialties, the documents list the number of wives, children and "slave girls" each had. A photo is also included.

It was not possibly to verify the personal information, but Iraqi officers who found the file said they believe it is genuine.

Two men from Kosovo refused to fight and asked to move to Syria. One said he had head pain.

This Washington Post illustration shows an English translation of the Isis file. Photo / The Washington Post
This Washington Post illustration shows an English translation of the Isis file. Photo / The Washington Post

Of the more than 4000 foreign fighters who have left European Union nations for Iraq and Syria, around a third have returned, according to a report from The Hague-based International Centre for Counter-Terrorism.

About 14 per cent have been confirmed dead, while the rest remain overseas or their whereabouts are unknown.

"People say that they are the most motivated, but there are plenty of foreign fighters that went and found that the Isis experience wasn't what they thought it would be; they thought it would be a great adventure," said Aymenn al-Timimi, an analyst specialising in militant groups who has compiled an online database of Isis documents, some of which indicate similar issues of morale.

The organisation keeps meticulous records, leaving clues to its inner workings as the fighters are ejected from territory.

Iraqi counterterrorism forces discovered the documents in a house in Mosul's al-Andalus neighbourhood that was being used as an administrative base for the Tariq Bin Ziyad battalion.

The militants were seen removing documents and computers from the building, according to neighbours, before they set fire to the building as Iraqi forces retook the area, said Lieutenant Colonel Muhanad al-Tamimi, whose unit found the documents unscathed in a desk drawer.

"Those foreign fighters are the most furious fighters we ever fought against," he said.

"When those fighters refuse to fight it means that they've realised this organisation is fake Islam and not the one they came for."

Iraqi troops faced a barrage of suicide car bombs and fierce resistance during the first month of their operations to retake Mosul last year. However, after pausing to reorganise, the forces have made rapid progress on the eastern side of the city this year.

Edwin Bakker, a research fellow at the International Center for Counter-Terrorism and a professor of counterterrorism at Leiden University, said that fighters from Western European countries are largely known to intelligence agencies, but that there is less information on those from countries such as Bosnia and Kosovo.

With open borders in Europe, these fighters might return home and stage attacks on the continent, he said. But warnings of a "tsunami"of returning foreign fighters are exaggerated, he said.

"We shouldn't underestimate the numbers that have gone to live there and die there," he added.