A social media account belonging to the worldwide brigade of mainly European volunteers fighting with the Kurdish-led Syrian Democratic Forces (SDF) showed a video of the man being questioned.
The man, who said his name was Anwar Miah, was reportedly seized by Kurdish forces near the Iraqi border.
"I'm a doctor", he said when asked when he joined the Islamic State of Iraq and the Levant (Isil).
Speaking with a Birmingham accent, the man tells his captors he is called Anwar Miah and is a qualified pharmacist from Birmingham.
"The areas that I worked in were controlled by Daesh [Isis], but I have worked with the general people". They were controlled by Daesh but I can't do anything about that.
Mohammed Anwar Miah, 40, from Sparkhill, Birmingham, was filmed by the Syrian Democratic Forces (SDF) being detained near the town of Hajin in Deir Ezzor province, eastern Syria. "All my work was with the public".
The director of Kings College London's International Centre for the Study of Radicalisation Shiraz Maher said the man's alleged detention raised questions over how many foreigners remain in the region with the terror group.
A video of his capture has surfaced on Twitter and shows Miah saying he has lived in Syria for almost four years.
"One of the big questions and dilemmas for security agencies, after Islamic State lost Raqqa and Mosul, is: who died?"
At least three other British men who are accused of being members of IS are now in Kurdish and United States custody in Syria.
Miah's arrest brings the number of British citizens detained by Kurdish forces in northern Syria to four.
Other foreign fighters held by the SDF include Londoners Alexander Kotey and Elshafee ElSheikh.
The pair's capture became the subject of controversy after it emerged the Home Secretary had not sought a guarantee that they will not face execution if extradited to the U.S. for terror crimes.
"Anyone who has travelled to these areas against United Kingdom government advice, for whatever reason, is not only putting themselves in considerable danger but also under suspicion".