A British man who joined Kurdish forces to fight the Islamic State group in Syria has been arrested attempting to cross the Iraqi border.
IT worker Jac Holmes, from Bournemouth, had been fighting with the main Kurdish force in the northern Syria.
He was detained with Joe Akerman, from Halifax, and Irish citizen Joshua Molloy in an area controlled by the the Kurdistan Regional Government.
Foreign Office officials are due to visit him in prison.
The UK government strongly warns against travelling to the region.
The three men had been returning to the UK at the time of their arrest, the BBC understands.
The group had been waiting some weeks waiting to cross the border, which was closed. They were detained five days ago after attempting to cross another part of the frontier.
It is not known on what grounds they were detained.
Kurdish rights activist Mark Campbell said Mr Holmes was "very tired and in need of rest and recuperation".
"He's done a brave thing in the view of a lot of people and then to have to face all this is just an extra burden.
"He's a very resourceful young man, I think he'll be bearing up under the stress. And probably hoping things will work themselves out."
Despite having no military experience, Mr Holmes first entered Syria with the People's Protection Unit, or YPG, aged 22, in January 2015.
He has previously told the BBC his interest in Syria began in 2011 with his attention gradually shifting to the struggle of the Kurds in the north as IS started to target them.
He started following Kurdish social media accounts, as well as those of Western volunteers fighting with Kurds.
During his first spell in Syria he was shot through the right shoulder while fighting. He returned to the UK in August but said he willing to return to Syria.
More than 250,000 people have been killed since the civil war in Syria began in 2011.