The German government plans to introduce legislation enabling authorities to strip Germans with dual nationality of their German citizenship if they fight for a terror group.

Interior Ministry spokeswoman Eleonore Petermann said Monday that the new rule will be applicable to over-18s and "for reasons of constitutional law" can only apply to future cases and can't be used for retrospective cases.

Officials said the legislation will be drawn up soon but didn't offer a precise timetable.

German law already allows for dual nationals to be stripped of their German nationality if they volunteer for their other country's armed forces without the consent of German authorities.

The move comes as Germany and other European countries consider what to do about those citizens who fought for the Islamic State group.