German Chancellor Angela Merkel has warned Turkey's president that comparing German officials to Nazis must stop, "no ifs, no buts".

Axar.az reports that Mrs Merkel said if Recep Tayyip Erdogan continues, more campaign events by Turkish politicians may be banned.

German authorities have cancelled rallies designed to woo ethnic Turkish voters ahead of a key referendum on expanding Mr Erdogan's powers.

"You are right now employing Nazi measures," Mr Erdogan said on Sunday. "When we call them Nazis they (Europe) get uncomfortable. They rally together in solidarity. Especially Merkel," he said in a televised speech.

Mr Erdogan appeared to be addressing Mrs Merkel personally when he suggested that Germany was "employing Nazi measures", using the informal "you" in Turkish.

"My remark that the Nazi comparisons from Turkey must stop still applies. No ifs, no buts," Mrs Merkel said at a news conference in Hannover on Monday with visiting Japanese Prime Minister Shinzo Abe.