Turkish Foreign Minister Mevlut Cavusoglu met with his Swiss counterpart Thursday after having to call off a campaign appearance amongst diaspora voters, in a bitter row between Ankara and Europe.
President Recep Tayyip Erdoğan has accused Germany and the Netherlands of behaving like Nazis for halting some rallies by Turkish ministers, comments that both countries have called unacceptable.
The incident comes almost a year after the European Union signed a deal with Ankara to stop the flow of refugees in exchange for billions of euros in financial aid to Turkey and the lifting of short-term visa requirements for Turks.
Turkish President Tayyip Erdogan attends a ceremony marking the 102nd anniversary of the Battle of Canakkale in Canakkale, Turkey, March 18.
He added: "On the one hand you say democracy, on the other you are putting pressure on Turks".
The Office of the Attorney General told AFP it had "concrete suspicions (of espionage) against the Turkish community in Switzerland (by) a political intelligence service", and said it had opened its probe on March 16, after receiving a green light from the Swiss government.
Close to 70,000 Turkish citizens live in Switzerland, according to Swiss government statistics, while the Turkish embassy's website refers to 130,000 Turkish nationals. "We can not accept their attitude on banning our foreign minister's entry to their country", Erdogan said.
Swiss prosecutors are investigating claims Turkish citizens living in Switzerland are being spied on.
Turkey's dispute with the European Union erupted earlier this month after Germany and the Netherlands blocked Turkish ministers from holding rallies to secure a "Yes" vote in the next month's referendum on expanding Turkish President Recep Tayyip Erdogan's powers.