John Cantlie said the group was "dug in for the fight" and that US-led air strikes against them would not be successful.
He said: "Anyone hoping for a nice neat surgical operation without getting their hands dirty is in for a horrible surprise once it gets underway."
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John Cantlie has always appeared dressed in an orange jumpsuit, a reference to Guantanamo Bay
Cantlie added that the brutal beheading's broadcast online "were a win win" for IS.
He said: "If these executions force public outcry or a policy change, that is a huge victory.
"And if they only goad our governments into dropping more bombs and spending millions more dollars, making our countries weaker in the process, that is a victory too."
The freelance photojournalist has been held captive for nearly two years.
It says: "Four of my cellmates have already been executed by the Islamic State in the most visceral way possible after the British and American governments apparently made a joint decision not to discuss terms for our release with our captors.
"And now, unless something changes very quickly and very radically, I await my turn.
"I've had to watch as James (Foley), Steven Sotloff, David Haines and Alan Henning, walked out of the door, one every two weeks since August 18, never to return, knowing they were going to be killed and going to their deaths."
The 16-year-old schoolgirls from Chorlton, Manchester, followed their brother who had also gone to fight in Syria. They are now married to ISIS fighters, and told a reporter for their local paper that they spend most of their time indoors, leaving only with their husbands. Both twins, the daughters of Somali refugees, had achieved excellent GCSE results, 23 grades A*-C between them at Whalley Range High School for Girls. A twitter account linked to one of the twins shows a woman in a burka, with an AK47. It appears the account has since been removed.
Khadijah Dare aka Muhajirah fi Sham(02 of10)
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Khadijah Dare, a mother of one originally from Lewisham, has engaged in active recruitment of women for Isis. She left Britain in 2012 to live in Syria with her Swedish husband.Writing on Twitter under her name Muhajirah fi Sham, which means immigrant in Syria, Dare praised the killing of US journalist James Foley, saying: “Any links 4 da execution of da journalist plz. Allahu Akbar. UK must b shaking up ha ha. I wna b da 1st UK woman 2 kill a UK or US terorrist!(sic)”.In a recruitment video for the group, the 22-year-old can be seen firing an AK47, calling on Brits to come and fight. “Instead of sitting down and focusing on your families or focusing on your studies, you need to stop being selfish because time is ticking," she said.
Aqsa Mahmood aka Umm Layth (03 of10)
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Umm Layth was a prolific tweeter until she was identified in the press as 20-year-old Glaswegian Aqsa Mahmood.In her tweets, she urged Muslim men and women who could not come to fight to instead commit terrorist atrocities at home, praising the brutal murder of Drummer Lee Rigby in Woolwich, the bombing of the Boston Marathon and the shooting of soldiers at Fort Hood in Texas. "If you cannot make it to the battlefield, then bring the battlefield to yourself," she tweeted.Mahmood, who is now married to an Isis fighter, attended the prestigious Craigholme School and was studying radiography at Glasgow Caledonian University when she left for Syria. Her family reported her missing to police in November 2013.Despite praising al Qaeda terrorists and encouraging more attacks, her tweets betray her Westernised roots. One asks for someone to "make a Hijrah [pilgrimage] from Scotland already and bring me Irn-Bru.” She also tweeted with delight at receiving European food, including Pringles crisps and Nutella.
A close friend of Aqsa and a Brit of Somalian heritage, Khanssaa is described on Twitter as the "cook of the house" in Raqqa where several girls live. She tweets them offering up Nutella pancakes. Unlike many of the other girls who have tweeted about how their families disapprove of their mission to Syria, Khanssaa said she is following in the footsteps of her father who left her family to fight a holy war, though she does not specify where.
My father left for jihad with 3 children under the age of 4 & a wife 9 months pregnant and brothers stay behind because its hard to leave..?
With the black flag of Isis as her profile picture, Umm Anwar, who also goes by the name Umm Farris, is one of the four British girls married to an Islamic State fighters who has only recently been identified by researchers. She is believed to be based in Raqqa, and recently said she was surprised to discover a ‘Yazidi slave girl’ from Iraq in a home she visited. As well as retweeting praise for Islamic State fighters and the Caliphate, she mentions shopping and joking with her friends in the city while her husband fights.
Though much of her account extols the virtue of jihad, the third member of the British girl gang in Raqqa peppers her tweets with English slang, like ‘ain’t’ and calls her fellow ISIS wives ‘babesss’. Going under the name 'Black Banners' on Twitter, where her profile picture includes Osama Bin Laden, she suggested she is the second wife of a fighterShe tweets about being “bored” in Raqqa and asks her friends repeatedly to meet up and visit her. Her twitter also includes retweets of beautiful pictures and Vines, including a sunset at the Golden Gate bridge, San Francisco, and a comedy sketch about accidentally dropping a cookie in milk.
UkhtiB(07 of10)
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A close friend of Anwar, the pair joke about their shopping habits on Twitter, arrange lifts, drink smoothies and cook each other food. Her background is unclear, but she hints that her family disapprove of her being in Syria, tweeting: “Your family will be the biggest test for you once you make Hijrāh. They're either with you or without you.”Much of her feed consists of retweets of local fighters and of Islamic sayings, as well as graphic pictures of the dead from Iraq, Syria and Gaza.
Umm Talib(08 of10)
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The fourth member of the group of girls in Raqqa, who calls herself Qad Af-Iahal Shuhada, has a son with her, and is believed to be from London or the south of England, having tweeted about leaving her Oyster card in the pocket of her abaya, a type of female Muslim covering, while she put it in the watch. A foodie, she recently retweeted a recipe for Vietnamese chicken with avocado and lemongrass spring rolls, then messaged her friend to tell her she was cooking for them. Other tweets include a picture of the girls out for dinner in Raqqa, eating hummus and pita with chilli and vegetables.
One of the most prolific tweeters amongst the women in Isis is a British 18-year-old who goes under the twitter handle @UmmKhattab, who has tweeted about previously being based in the town of Manbij, close to Aleppo, tweeting sunsets from the rooftops, but has recently moved to Raqqa. "Best thing ive done in my 18 years in this world is come to the blessed land of shaam and leave Britain the land of kuffar," she posted in June.And she tweeted a dim view of the UK's plan to strip returning jihadists of their citizenship. "Uk government are funny im not returning to ur dirty society which has no moral values y'all r all uncivilised and need islam to liberate u," she wrote.
The black flag is not only limited to iraq and shaam in sha Allah it shall fly over 10 downing st
Formerly a rock musician in a local band, the 45-year-old mother-of-two from Chatham, Kent, is believed to have converted to Islam to marry a British Isis fighter Junaid Hussain. The couple are reported to have moved to Raqqa, leaving her children behind.Her Twitter account under the name Umm Hussain al-Britani, contains threats like "You Christians all need beheading with a blunt knife and stuck on the railings at Raqqa... Come here I'll do it for you."