Germany's coalition government says it will introduce legislation that will allow authorities to strip people with dual nationality of their German citizenship if they fight for a foreign armed group.
The proposed rule will apply to people over 18 and, for reasons of constitutional law, only to future cases, a spokeswoman for the interior ministry said on Monday.
"There are plans to introduce a legal amendment, for Germans with multiple nationalities who fight for a terror militia, to lose their German citizenship," said Eleonore Petermann.
The move comes as Germany and other European countries consider what to do about citizens who fought for the Islamic State of Iraq and the Levant (ISIL or ISIS) group and were captured in Syria.
More than 900 people from Germany left the country to join armed groups in Syria and Iraq, according to a 2017 report by the Soufan Group, a New York-based security consultancy. About a third have returned home.
There are more than 1,000 ISIL prisoners from Europe in detention facilities overseen by the US-backed Syrian Democratic Forces in eastern Syria.
Washington has urged its European allies to repatriate the fighters, but Germany's Foreign Minister Heiko Maas previously said German ISIL prisoners could only "return if it is ensured they can immediately be taken into custody".
Still, the fact that the planned legislation cannot be applied retroactively means the proposal is unlikely to ease the German government's dilemma on how to handle former fighters.
Berlin hopes the legislation will have a "preventive effect" and discourage citizens from joining armed groups.
Officials said the proposals would be drawn up soon but did not offer a precise timetable.
German law already allows for people with dual citizenship to be stripped of their German nationality if they volunteer for their other country's armed forces without the consent of authorities in Germany.
The planned amendment would add participation in combat for a foreign armed group to the existing law.
Meanwhile, Steffen Seibert, spokesman for Chancellor Angela Merkel, rejected claims that the amendment would leave dual nationals feeling like "second-class citizens".
"We must not forget what this is about," he said.
"This is about concrete participation in combat operations for a terror militia abroad," he said.
Merkel's government had committed to examining such an amendment during their coalition negotiations last year, he noted.
Bernd Baumann, a politician with the opposition far-right Alternative for Germany party, said the proposed legislation came "much too late" and would not help with ISIL fighters returning from Syria and Iraq.
Stephan Thomae, a politician with the opposition Free Democrats, described the plan as "pure display window politics" and said the government should concentrate on taking a clear position on how to deal with captured ISIL fighters.
Other countries have made similar moves.
Neighbouring France has had a process since the 1990s for stripping French citizenship from dual nationals who carry out "acts of terrorism", though it has rarely been used.
The UK recently sparked controversy when it decided to strip a teenage girl who left to join ISIL in Syria of her nationality, arguing that she could obtain Bangladeshi citizenship through her mother, a claim the government in Dhaka has rejected.
