A German regulator has banned the sale of smartwatches aimed at children because they can be used as spying devices.

The agency said that the watches, aimed at children between the ages of five and 12, contained a SIM card and a “limited telephony function” that could be set up and controlled using an app.

“Via an app, parents can use such children’s watches to listen unnoticed to the child’s environment and they are to be regarded as an unauthorised transmitting system,” said Jochen Homann, president of the Federal Network Agency, in a statement, according to the BBC. “The watches are regarded as unauthorized transmitting equipment. According to our research, parents’ watches are also used to listen to teachers in the classroom.”

The agency is encouraging parents to destroy the smartwatches they already own and is asking schools to “pay more attention” to such watches among students, the BBC reports, and encouraged parents to destroy the ones they already own. The agency appears to be taking particular issue with the devices’ abilities to transmit audio from its surroundings.