A video of his capture has surfaced on Twitter and shows Mr Miah saying he has lived in Syria for nearly four years.
It is believed he is being held in a prison in northern Syria, guarded by US special forces.
In the video, Mr Miah is blindfolded and says he has been working as a medic in IS territory for the last four years.
He said: “I’m a qualified pharmacist from the UK. I’ve been working in the hospitals since I came.”
At least three other British men who are accused of being members of IS are currently in Kurdish and US custody in Syria.
They include two Londoners, Shafi El-Sheikh and Alexanda Kotey, who are alleged to be members of the notorious group dubbed the The Beatles. The pair were captured in January.
US officials believe the cell they belonged to beheaded more than 27 Western hostages and tortured many more.