Analysts on Thursday cast doubt on the authenticity of thousands of documents reportedly leaked from the Islamic State jihadist group, pointing out mistakes and uncharacteristic language.

The trove of documents, which includes the names, addresses, phone numbers and family contacts of IS jihadists, was handed over to Britain's Sky News by a disillusioned former member, the broadcaster said Wednesday.

Syrian opposition news website Zaman al-Wasl said there were thousands of repetitions in the leaked documents and the names of only 1,700 people could be identified in the 22,000 documents.

The files include forms that IS recruits reportedly had to fill out in order to join the organisation and contain information on nationals from 51 countries.

There were several inconsistencies in the language of the forms that raised concerns, experts said.

The Arabic name for "The Islamic State in Iraq and Syria," IS's previous nomenclature, is written in two different ways, including one that is not consistent with past practice.

Files documenting the deaths of IS militants use the words "date of killing" instead of the typical jihadist term "martyrdom."

Romain Caillet, an independent jihadism expert, also noted that some documents feature a second, circular logo not previously used on IS files.

"There would be big alarm bells for me, because when I've seen inconsistencies like that in the past they've been on really shoddily made forgeries," Charlie Winter, a researcher at Georgia State University, told AFP.

The biggest concerns, Winter said, were the different names, logo, and grammatical mistakes that he described as "very much out of character" for IS documents.

"With something as important as this, it's important to look at it with as suspicious, discerning, and cynical an eye as possible," Winter said.

'Less sophisticated'

Journalist and jihadism expert Wassim Nasr said the inconsistencies call into question the authenticity of at least some of the documents.

"Maybe some of the information is real, while the layout was fabricated to sell the information at a high price to different buyers," Nasr posted on Twitter.

Copies of the documents broadcast by Sky News showed that recruits would have to answer 23 questions including on their blood type, mother's maiden name, "level of sharia understanding" and previous experience.

Some of the names in the documents are of fighters who have been already identified, such as Abdel-Majed Abdel Bary, a former rapper from west London who once posted an image of himself on Twitter holding a severed head.

Others named include Junaid Hussain, a cyber-operative for IS from the British city of Birmingham, and 21-year-old Reyaad Khan, who appeared in a recruitment video. Both were killed last year.

Dalia Ghanem-Yazbeck, a jihadism researcher at the Carnegie Middle East Centre in Beirut, told AFP the registration forms "appear less sophisticated" than documents leaked in the past.

But she said some of the language mistakes and other red flags could be attributed to IS's nascent bureaucracy in 2013, when most of the forms were reportedly filled in.

"These registration forms were collected from fighters at the end of 2013, when IS was at an early stage in its state-building capacities," Ghanem-Yazbeck said.

"In that regard, what is interesting is the fact that even then, they had managed to recruit so many fighters -- at least 1,736 registration forms," she said.

If real, the leak "is bad for the reputation of 'total allegiance' of IS members, and bad for the organisation" because it "shows its incapacity to protect such documents," she said.

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