A German author given rare access to territory run by Islamic State has told the BBC that the group is stronger, more brutal and harder to confront than he had expected.
Juergen Todenhoefer spent six days in the IS city of Mosul in Iraq, travelling there via Raqqa, in Syria.
Mr Todenhoefer said he found IS followers highly motivated and supportive of the group's brutality.
He said the spread of fighters meant they were hard targets for air strikes.