A German official on Tuesday accused Turkey of carrying out "unacceptable" spying on alleged sympathisers of the movement led by exiled preacher Fethullah Gulen, blamed for a failed coup last year.

"It is notable with what intensity and ruthlessness the people living abroad are being investigated," said Boris Pistorius, interior minister for the northern German state of Lower Saxony, saying that Ankara had asked Berlin to help spy on 300 alleged Gulen supporters.

"It's intolerable and unacceptable," he told a press conference.