Islamic State (IS) militants fighting in Iraq and Syria have released a video which they claim shows British journalist John Cantlie in captivity, saying he will soon reveal “facts” about the group to counter its portrayal in Western media.

The Sunni militant group, which controls territory in Syria and Iraq, has already beheaded two American journalists and one British aid worker in recent weeks in what it said was reprisal for United States air strikes against it in Iraq.

• Journalist beheaded by Islamic State militants
• Journalist hostage freed in Syria

But in the new roughly three-minute video posted on social media sites, the man identified as Cantlie appears in good health and promises to “convey some facts” in a series of “programs”, suggesting there would be further instalments.

“Now, I know what you’re thinking. You’re thinking, ‘He’s only doing this because he’s a prisoner. He’s got a gun at his head and he’s being forced to do this.’ Right?” the man in the video, wearing an orange shirt and closely-cropped hair, said.

Support for the Islamic State is on the rise. Photo: Getty

“Well, it’s true. I am a prisoner. That I cannot deny. But seeing as I’ve been abandoned by my government and my fate now lies in the hands of the Islamic State, I have nothing to lose.”

In the new video, titled “Lend Me Your Ears, Messages from the British Detainee John Cantlie”, the man identified as Cantlie said he was captured after arriving in Syria in November 2012.

He said he worked for newspapers and magazines in Britain including the Sunday Times, the Sun and the Sunday Telegraph.

“After two disastrous and hugely unpopular wars in Afghanistan and Iraq, why is it that our governments appear so keen to get involved in yet another unwinnable conflict?” the man says in the video.

“I’m going to show you the truth behind the systems and motivation of the Islamic State, and how the Western media, the very organisation I used to work for, can twist and manipulate that truth for the public back home.”

The man said other Western governments have negotiated for the release of their hostages but that the British and U.S. governments chose to do things differently.

British foreign secretary Philip Hammond said he had heard reports of a video on social media and said authorities would look closely at any material released online.

“These videos can be very distressing for the families of the individuals involved,” he said.