The Islamic State in Iraq and the Levant group (ISIL), 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 Cantlie appears in good health and promises to "convey some facts" in a series of "programs," suggesting there would be further installments, Reuters reported.
"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, 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."
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 told reporters during a visit to Copenhagen.
US President Barack Obama has been trying to build an international coalition to destroy ISIL group which has exploited the chaos of Syria and Iraq to seize swathes of territory in both countries.
The United States has already carried out scores of air strikes against the group in Iraq and Obama said in a policy speech he would not hesitate to strike it in Syria as well.