BERLIN, July 20. /TASS/. Germany will review its policy towards Turkey and may cut investments to Ankara as bilateral relations have deteriorated, German Foreign Minister Sigmar Gabriel said on Thursday.

Turkish-German relations have been severely strained after Turkey detained six human rights activists, including a German citizen Peter Steudtner, earlier this week accusing them of assisting the activity of terrorist groups.

"We need to review our policy towards Turkey," Gabriel said. "The situation in Turkey is not transparent and we should call a spade a spade," he said. "We expect that Ankara will return to European values."

"We cannot advise investing in a country that lacks security and where enterprises are equaled to terrorists," the minister said. "I don’t see how the German government can guarantee safety of German investments in Turkey when they may face measures taken due to political motives."

Gabriel said it is impossible to continue talks between the European Union and Turkey on setting up a customs union under the current circumstances. "I cannot imagine talks on expanding the EU customs union when Turkey detains EU citizens without any grounds," he said.

"We should discuss the future of framework investment contracts, loans and providing German assistance," he said. In the coming days, Gabriel plans to discuss relations with Turkey with his EU colleagues.

German authorities have also decided to toughen recommendations for their citizens on trips to Turkey. "The situation around (human rights activist Peter) Steudtner shows that German citizens in Turkey cannot be protected from arbitrary detention," he stressed. "Any German citizen may fall victim" to Turkey’s arbitrariness, he noted.

Gabriel called the detention of six human rights activists in Turkey "well-planned steps," describing accusations against them as "unfounded and far-fetched." He called on Turkey’s authorities to resume dialogue "based on European values."

He said the detained German citizen attended a human rights seminar and was not an expert on Turkey. Steudtner doesn’t have any acquaintances among local politicians and he has never made any publications in local media.

German Chancellor Angela Merkel sided with the foreign minister. "Chancellor Merkel considers that the measures announced by the foreign minister on Turkey in the light of recent events are necessary and inevitable," German Government Spokesman Steffen Seibert wrote on Twitter.

Turkish Presidential Spokesman Ibrahim Kalin said the latest statements of German politicians are linked to the current political situation in the country, which is gearing up for parliamentary elections in September. "We will not accept these statements and expect that soon Germany will abandon them."

The spokesman stressed that Turkey wants to maintain relations with Germany "which are at a good level now." He assured that "there is no danger for German citizens traveling to Turkey."

Relations between Turkey and Germany started worsening last year when the German parliament unanimously passed a resolution calling the killings of Armenians in the Ottoman Empire "genocide." The Turkish leadership demanded that Berlin should publicly distance itself from the resolution. Besides, Ankara was angered by a satire show on Germany’s TV insulting Turkish President Recep Tayyip Erdogan.

This spring, Turkish politicians planned to make a tour of European countries and address their countrymen ahead of the referendum on a constitutional reform boosting Erdogan’s powers. The German government said their goal was to carry out propaganda in Germany and banned them from holding these events, sparking criticism in Ankara, which accused Berlin of using "Nazi methods" against Turks.

Germany is home to around 1.5 million natives of Turkey, who still hold a Turkish passport. At the referendum, nearly two thirds of them voted in support of Erdogan’s reforms, alarming local observers and politicians.

Amid tensions between Ankara and Berlin, analysts voice concerns over the fate of a deal on refugees reached between the EU and Turkey. At the height of the migration crisis, hundreds of thousands of them entered the EU through Turkey’s territory and moved to Germany, what led to a drop in Merkel’s rating and strengening the positions of the right-wing Alternative for Germany party.