By Michelle Martin
BERLIN (Reuters) - Turkey's behavior is "unacceptable" and Germany has a duty to protect its citizens and companies but also wants to maintain strong ties with Ankara, German Chancellor Angela Merkel's chief of staff said on Sunday.
Relations between the NATO allies have deteriorated since Turkey arrested six rights activists, including one German, two weeks ago as part of a wider crackdown since last year's failed coup against President Tayyip Erdogan.
"We want to have good relations with this big and important country but that's only possible if Turkey is and remains a state under the rule of law," Peter Altmaier told newspaper Bild am Sonntag.
"Turkey's behavior is unacceptable," Altmaier said when asked about Turkey barring German lawmakers from visiting soldiers at a base in Turkey, the arrest of Germans and Erdogan's recent comments on Germany.
Tensions are already high between the two countries following the arrest of a Turkish-German journalist and a pullout of German troops from a Turkish air base.
Berlin wants German rights activist Peter Steudtner and journalist Deniz Yucel to be freed as there is no apparent reason for their arrests and the government is taking every opportunity to get consular access to them, to talk with the Turkish government and with Erdogan, Altmaier said.
The German government is monitoring developments in Turkey closely and will decide on sanctions if necessary, he said.
Germany has increased pressure on Turkey in the past few days, threatening measures that could hinder German investment there and saying it is reviewing Turkish applications for arms projects.
On Saturday, German Foreign Minister Sigmar Gabriel sought to reassure the three million people in Germany of Turkish descent in a letter published in the Bild newspaper that they belonged and were not the target of changes to policy on Turkey.
German President Frank-Walter Steinmeier told broadcaster ZDF the Turkish diaspora in Germany meant ties with Turkey were "especially important" and they were probably suffering the most when they saw Ankara tearing down bridges built with Germany over many years.
Manfred Weber, who heads the conservative bloc in the European Parliament, told Der Tagesspiegel newspaper that Turkey was moving away from the European Union and its values, adding: "The accession talks, which were an illusion from the outset, must be completely terminated."
The vast majority of Germans are unhappy with German policy towards Turkey, an Emnid survey for Bild am Sonntag newspaper showed. Some 76 percent said the government was not defending itself enough against Erdogan, while 12 percent disagreed.
Germany has warned citizens who travel to Turkey they do so at their own risk. The Emnid survey, published on Sunday, also found 49 percent of Germans do not think they can go on holiday in Turkey anymore while 44 percent think they can.
The head of foreign trade at Germany's chambers of commerce (DIHK) told Welt am Sonntag newspaper that increasing bilateral tensions were creating uncertainty for German firms and would likely push trade volumes down by about 2 billion euros ($2.3 billion).
($1 = 0.8578 euros)
(Reporting by Michelle Martin; editing by Jane Merriman)