Twelve Germans are being held as political prisoners in Turkey, according to Berlin, with the latest two arrests announced Friday.
Relations between Berlin and Ankara have been in a downward spiral since last summer, when a failed coup against Turkish President Recep Tayyip Erdoğan sparked a crackdown on civil liberties and mass arrests of the political opposition, activists and journalists, including German citizens.
Germany has accused Erdogan of attempting to silence his critics at home and abroad. Erdogan, in turn, has called on voters in Germany to reject the country's biggest parties in next month's election.
Steffen Seibert, Merkel's spokesman, warned that Germany expects "German citizens who are imprisoned for incomprehensible reasons, be released."
According to Germany's Foreign Ministry, its consul in Izmir was informed by nongovernmental authorities of the latest arrests and hasn't been able to make contact with the two yet.
Speaking at her annual summer news conference this week, Merkel said Turkey's jailing of Germans was further damaging already fraught ties between the two countries.
"We must see how things develop, but we are calling now, very clearly, for the release of those who are imprisoned," she said.
"Several German citizens are being held in prison, which we believe is not justified. We therefore decided to take a new direction in our policy toward Turkey."
Deniz Yucel, a German-Turkish journalist, was arrested in Turkey in February on charges of terror propaganda. He has been held for 200 days, according to German authorities.
In July, Turkish authorities arrested German human rights activist Peter Steudtner
and nine others, charging them with "committing crimes in the name of a terrorist organization without being a member."
Earlier this month, a prominent Turkish-German writer who has been critical of Erdogan was detained while on vacation in Spain.
Dogan Akhanli, who lives in Cologne, was released after a court hearing on the condition he remain in Madrid, according to his attorney.
Akhanli's arrest prompted German government accusations that Turkey is using Interpol, an international police organization, to hunt down Erdogan's political opponents abroad.
Germany threatens trade and travel restrictions
Germany has changed its tactics over Turkey in recent months, threatening to impose travel and trade restrictions if journalist Yucel and activist Steudtner aren't released from prison.
Last month, German Foreign Minister Sigmar Gabriel warned Germans against traveling to Turkey and suggested that the German government would review corporate investments in Turkey.
"Someone who detains law-abiding visitors to their country on the basis of outlandish, indeed absurd, accusations and throws them into prison has left European values behind," Gabriel said in July, calling for Steudtner's release. "We cannot continue as before."
A few weeks earlier, Turkey's Foreign Ministry criticized an art installation in Berlin depicting Erdogan as a dictator that coincided with the G20 summit in Hamburg, calling it "a new example of rising racism and xenophobia in the country."
Soon after, the Turkish government blocked German lawmakers from visiting German troops stationed in Turkey participating in NATO operations in Syria.
Earlier this year, German officials prevented top politicians, including Erdogan, from addressing Turkish rallies in Germany in the lead-up to an April referendum that handed Erdogan sweeping new powers.
In response, Erdogan likened the German government to that of Adolf Hitler. "I thought that Nazism was over in Germany, but it turns out that it is still going on," he said. "It is still going on, it is clear."
Merkel warns Erdogan over election
Relations with Turkey are a key issue in the run-up to Germany's federal elections, and some 3 million people with Turkish roots live in Germany.
Earlier this month, Erdogan called on voters of Turkish origin to boycott the two biggest parties -- Merkel's Christian Democrats and the Social Democrats -- along with the Green Party in the election, describing them as "enemies of Turkey," according to CNN affiliate NTV.
The call drew a fierce rebuke at the time from Merkel, who warned Erdogan against interfering in the election.
Germans go the polls September 24, with Merkel widely expected to secure a fourth term.