German officials have sought to maintain a calm, patient tone, even while expressing concerns over Mr. Erdogan’s steady expansion of his powers and his firing of tens of thousands of people regarded as political opponents. Since the coup attempt, several hundred Turkish diplomats, soldiers and others have applied for political asylum in Germany.
Last month, Germany said it would withdraw its forces from a military base in southern Turkey after Mr. Erdogan’s government refused to guarantee visits to forces there by German lawmakers, which lawmakers are required to do under the German Constitution.
The tensions came to a new head this week after the Turkish authorities decided to hold Mr. Steudtner under arrest. He was detained on suspicion on July 5 with five others, including Amnesty International’s Turkey director, in a raid on a hotel where they were attending a digital security workshop. The Turkish authorities have accused them of having links to terrorist groups.
“The case of Peter Steudtner shows that German citizens are no longer safe from arbitrary arrests,” Mr. Gabriel said. German authorities said nine other German citizens, including two journalists, Deniz Yucel and Mesale Tolu, were being held by Turkey.
Mr. Gabriel said the Germans had been accused without evidence. He cited their detentions as “examples of the absurd accusations of terror propaganda that obviously are only meant to serve to silence every critical voice in Turkey.”
In a strongly worded statement that nonetheless fell short of an outright travel warning, the German Foreign Ministry urged Germans traveling to Turkey to exercise caution and register with the German Embassy in Ankara, or one of Germany’s consulates in Turkey, “even for short trips.”
The German government will consider “further measures” in the coming weeks, after discussing the future of financial aid for Ankara with its European Union partners, in the context of the long-stalled talks about Turkey’s potentially joining the bloc, Mr. Gabriel said.
In Turkey, ministers lashed back, with Mr. Cavusoglu accusing Germany of maintaining an “unacceptable, one-sided attitude.”
Hours after Mr. Gabriel made his statement, prosecutors in the western city of Celle said they had arrested a Turkish man on suspicion of belonging to a terrorist organization and working as a regional leader for the P.K.K. in Europe.
Roughly four million Germans vacationed on Turkish beaches last year, an important source of income for the country. Additionally, an estimated three million people living in Germany either hold Turkish citizenship or are descendants of Turkish migrants, many of whom were invited to work in factories in West Germany after World War II.
On Wednesday, the German weekly newspaper Die Zeit reported that Turkish authorities had handed Berlin a list of 68 German companies they accused of having links to Mr. Gulen, who lives in self-imposed exile in Pennsylvania. The list included the chemicals giant BASF. Turkey has denied the report.
Mr. Gabriel said he could not advise firms to invest in a country where “even completely innocent companies are judged as being close to terrorists.” He added: “I can’t see how we as the German government can continue to guarantee corporate investments in Turkey if there is the threat of arbitrary expropriation for political reasons.”
Germany was Turkey’s top export destination in 2016, having bought $14 billion worth of Turkish goods, according to the International Monetary Fund. It was also the second-biggest source of imports to Turkey, at $21.5 billion, behind China, which exported $25.4 billion worth of goods to Turkey.
Michael Werz, a specialist on Turkey and trans-Atlantic relations at the Center for American Progress, a Washington policy research firm, said he feared that relations had “reached a point of no return.”
He said the travel warnings by Mr. Gabriel — who is generally viewed as favoring a softer stance toward Turkey — could “hurt the battered tourism section even more.”
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