The so-called Islamic State would respond to any intensification of Western air strikes by beheading more hostages, a man who claims to have defected from the terror group has warned.

The unnamed man - whose identity could not be independently verified - said the West was "stupid" in its outraged responses to IS's actions, which he said only encouraged the group to carry out more atrocities.

He said that the UK national nicknamed "jihadi John" was chosen to carry out the murders of British aid worker David Haines and two US journalists in order to "send a message" to Britain, which IS leaders believe may join the US in military action. And he said other Britons were working through the internet to recruit more Westerners to swell the group's ranks.

The man's comments were broadcast by BBC Radio 4's World At One, which said he told its reporter he was a Syrian fighting with another opposition group when the city of Raqqa was taken by IS. He joined up and was impressed by the extremists' efficiency and the "five-star lifestyle" they delivered their supporters, but decided to leave when he became disgusted by their brutality, including the arrest and torture of some of his friends.

He said that the militant Islamist group, which controls large areas in both Syria and Iraq, was holding foreign hostages in "special prisons", where they were questioned but not subjected to torture. And he claimed to have information that the Italian priest and peace activist Father Paolo Dall'Oglio - missing since being kidnapped in Raqqa last year - is still alive and in captivity.

Speaking through a translator, the man told World at One: "It is clear that the West are stupid. Whenever IS does something and the West gives speeches against it, IS becomes more determined to do similar things.

"IS is keen to plant its thought, its ideology in the minds of people. Even if it loses certain territory, it will be happy if only three or four people have been left in these territories believing in its ideology.

"If air raids intensify in Iraq and Syria, there will be more hostages beheaded."

He added: "I know where foreign hostages are held. I have been to their prison - I used to meet up with some of the Iraqi investigators sometimes at night, and they would tell me that they have been investigating or questioning certain hostages, certain captives, some from Iraq, some from Aleppo.

"As for British and French hostages, they are held in special prisons. Syrians are not allowed to make contact with them at all. Only Iraqis do the investigating and the questioning. At one time they were moved to an Iraqi prison for a whole month, but the organisation does not torture them.

"As for the American journalist that was killed by a British fighter - I cannot tell you his name or anything about him - the intention was to send a message to Britain. They decided to ask a British man to kill the American journalist because they had anticipated - and this turned out to be right - that Britain would eventually join the United States in the war."

Describing the activities of IS members, the man said: "Iraqis are mainly working in investigation and security, Tunisians are mainly military people, Saudis are jihadists seeking martyrdom.

"Foreigners, like British and French fighters, are mainly recruiters of new people. They are in contact with the internet 24 hours a day trying to recruit new people on the borders. There was one person who was able to recruit three or four new fighters every week."

Iraq's ambassador to London, Faik Nerweyi said that the man's account "just demonstrates how brutal (IS) are".

He told World At One: "It's a very difficult problem. It struck me how he said they are going to plant their ideology, their way of seeing religion the way they like into the minds of youth, and that's true. It's unfortunate."

Mr Nerweyi said that the provision of weapons and ammunition from the international community had made "a great difference in the attitude of fighting forces in all parts of Iraq", but called for the acceleration of discussions on military action against the jihadis.

"It looks like it will take time for the weapons that have been promised to come and also to make the fighters train on using these weapons, and put the whole plan of what to do next in terms of ejecting them out of Mosul and Tikrit and hopefully soon drive them out of Iraq completely," he said.

"That kind of negotiations and discussions between various participants of this whole coalition have to come, and should be a little bit faster in pace if we need them to go out of Iraq very soon."