BERLIN (Reuters) - Chancellor Angela Merkel on Friday said Germany should react decisively to Ankara’s detention of two further German citizens, amid growing calls for Berlin to issue a formal travel warning for Germans heading to Turkey.

FILE PHOTO: German Chancellor Angela Merkel meets Turkish President Recep Tayyip Erdogan on the eve of the G-20 summit in Hamburg, Germany, July 6, 2017. REUTERS/Michael Kappeler,POOL

Twelve German citizens are now in Turkish detention on political charges, four of them holding dual citizenship. Among these is German-Turkish journalist Deniz Yucel, who will have been in detention 200 days on Friday.

Under the circumstances, Merkel said she did not think it was appropriate to carry out further discussions with Ankara about its participation in a European Union customs union.

“We must react decisively,” Merkel told a business event in the southern city of Nuremberg, noting that Germany had already fundamentally revamped its relations with Ankara.

“Given the latest events, perhaps it is necessary to rethink them ever further.”

Germany was not officially informed of the two new detentions, which took place at Antalya airport on Thursday, leaving Berlin’s consulate in the coastal city of Izmir to learn of their arrest from “non-state sources”, Foreign Ministry spokeswoman Maria Adebahr told a news conference.

Many European citizens have been detained in Turkey over the past year, accused of involvement in last year’s failed coup against President Recep Tayyip Erdogan, whom many accuse of purging opposition under the cover of a crackdown.

“We’re trying to establish what they are charged with,” said Adebahr said. “We must assume that it’s a political charge, suspicion of terrorism, as with the others.”

Diplomats had not been able to contact them, she added, with Friday’s public holiday celebrating the Muslim festival of Eid a possible reason for delays in contacting officials.

Social Democrat Martin Schulz, Merkel’s main challenger in Sept. 24 elections, and other German politicians urged Berlin to issue a formal travel warning for Germans heading to Turkey.

The government in July urged German citizens to exercise caution if traveling to Turkey, but stopped short of issuing a formal travel warning.

Juergen Hardt, a senior member of Merkel’s conservatives, told Die Welt newspaper that a further tightening of the travel guidance “should be seriously considered”.

Cem Ozdemir, leader of the Greens party, told Bild newspaper he could no longer assure anyone they would be safe in Turkey.

“Erdogan is no president, but a hostage-taker,” Ozdemir told the daily newspaper Bild.

No comment was immediately available from the foreign ministry about whether it was considering a travel warning. About 3 million people with Turkish heritage or citizenship live in Germany.

Such a move could mark a significant setback for Turkey, which already saw the number of foreign visitors drop to its lowest level in nine years last year. Bookings from Germany accounted for some 10 percent of Turkey’s tourists this year.