British volunteers fighting against so-called Islamic State in Syria pose a domestic security threat to the UK, a foreign affairs think tank has said.
Since 2014, hundreds of people from Western countries, including the UK, have joined Kurdish rebel group YPG.
The Henry Jackson Society said YPG was a front for the banned PKK group and warned that those who joined it could be drawn into terror activity at home.
It said the government urgently needed to stop Britons joining the group.
The Home Office said those who return to the UK after taking part in conflicts in Syria or Iraq should expect to be reviewed by the police.
Read more at BBC News