ISLAMIC State militants have released a propaganda video that they said shows a British journalist in captivity saying he will soon reveal facts about the group to counter its portrayal in Western media.

The Islamic State, 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 US air strikes against it in Iraq.

But in the new roughly three-minute video posted on social media sites, the man identified as photojournalist John Cantlie appears in good health and promises to "convey some facts" in a series of "programmes," suggesting 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 says.

"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."

Foreign Secretary Philip Hammond said he had heard reports of a video on social media but had not seen it. He said authorities would look closely at any material released online.

"These videos can be very distressing for the families of the individuals involved," he told reporters.

The United States has already carried out scores of air strikes against the extremist group in Iraq and President Barack Obama said in a policy speech he would not hesitate to strike it in Syria as well. Islamic State have seized swathes of territory in both countries.

In the new video, titled Lend Me Your Ears, Messages from the British Detainee John Cantlie, the man identified as Mr Cantlie says he was captured by the Islamic State after arriving in Syria in November 2012.

"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.

"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."

Mr Cantlie, who has worked for the Sunday Times and The Sun, said other Western governments have negotiated for the release of their hostages but that the British and US governments chose to do things differently.

"I'll show you the truth behind what happened when many European citizens were imprisoned and later released by the Islamic State, and how the British and American governments thought they could do it differently to every other European country," he says.

"They negotiated with the Islamic State and got their people home while the British and Americans were left behind," he says.

Mr Cantlie was brought up in England. His grandfather designed the China Railways KF locomotive and his great-grandfather was the eminent Scottish surgeon Sir James Cantlie.

Mr Cantlie was previously taken hostage in July 2012 along with Dutch photographer Jeroen Oerlemans while working near the Syrian border with Turkey. They were released the same month after a group of "Free Syrian Army" fighters freed them. He said they were threatened with death unless they converted to Islam.

Mr Cantlie wrote in the Sunday Times that the group of about 30 militants had been made up of different nationalities, many British and none Syrian, and that the British jihadists had treated him the most cruelly in captivity.

In the video released yesterday the man identified as Mr Cantlie says: "Maybe I will live and maybe I will die. But I want to take this opportunity to convey some facts that you can verify. Facts that, if you contemplate, might help preserving lives."