But what is the motivation for people, usually revealed to be youngsters with little or no experience of conflict, to leave their homes in British towns and cities to do so?
One British man who is still in the UK has told the BBC in an interview that he feels it is his "obligation" to now go to Iraq or Syria.
We don't know his real name, or where he comes from in the UK, and during our TV interview his face is covered with a bright red scarf.
After he says hello, "Ahmed", as he wants to be known, speaks with purpose. He details why he wants to fight in Iraq or Syria.
"God has commanded for the Muslims to go and fight jihad", he says. (+ video) » | Ed Thomas | North of England Correspondent, BBC News | Friday, August 15, 2014
