The German Foreign Ministry has updated its warnings for citizens travelling to Turkey, underlining that Turkish officials are monitoring Virtual Private Networks (VPNs), Deutsche Welle Turkish service reported on Tuesday.
Using unverified VPNs in Turkey could be punished by the authorities, the ministry said.
It added that citizens who are active in pro-Kurdish associations in Germany could face detention and arrest in Turkey.
In October five German citizens were detained in Turkey on terrorism charges.
Dozens of other German nationals were also the subject of investigations in Turkey, including journalists Deniz Yücel and Meşale Tolu, who were jailed for months.
Turkey has been using its anti-terrorism laws against government critics since a failed coup in 2016.