“At the moment, a total of 20 accused and persons unknown are under investigation over suspicions that they worked as secret service agents on the orders of the Turkish government, and spied on followers of the Gulen movement,” said Germany's interior ministry in a written reply to a query by an opposition Left party lawmaker.

Erdogan blames US-based cleric Fethullah Gulen for a failed coup last July, and has launched a sweeping crackdown against his followers.

Gulen has denied all accusations, but more than 41,000 people in Turkey have been arrested over suspected links to his movement.

German prosecutors last month announced a probe into claims that Turkish agents have been spying on Gulen's followers in Germany.

They have also launched an investigation into the actions of Turkish imams suspected of acting as spies under Ankara's orders.

The interior ministry did not specify in its parliamentary reply if the 20 under probe included the imams.

But the investigation of the Muslim preachers had followed raids in February on the homes of four Turkish imams who are believed to have passed on information about Gulen followers to the Turkish consulate.

Relations between NATO allies Germany and Turkey have been strained by disputes related to last July's failed coup and a subsequent crackdown on alleged conspirators.

The row deepened after Germany and the Netherlands blocked campaign events by Turkish ministers in March ahead of a referendum vote to boost presidential powers.

In an angry response, Erdogan accused both countries of using “Nazi” methods, sparking consternation in Berlin.