BERLIN (Reuters) – Germany told its citizens on Thursday to exercise caution if traveling to Turkey and threatened measures that could hinder German investment there, in a sign of growing impatience with a NATO ally after the detention of rights activists.

The mass-selling daily Bild newspaper, citing government sources, also reported that Berlin was putting arms projects with Ankara on hold.

Foreign Minister Sigmar Gabriel highlighted alarm at what Berlin sees as the growing unpredictability of Turkish President Tayyip Erdogan. “Everyone can be affected. The most absurd things are possible,” he said in advice to travelers.

Gabriel broke off his holiday to deal with the crisis after Turkey arrested six human rights activists including German national Peter Steudtner on accusations of terrorism, the latest in a series of diplomatic rows.

Germany, Turkey’s chief export partner, called the allegations absurd.

“We need our policies towards Turkey to go in a new direction…we can’t continue as we have done,” Gabriel told reporters in unusually direct language touching on sensitive commercial matters including corporate investment guarantees.

The Turkish foreign ministry said it would make the “necessary response” to comments it described as one-sided.

Foreign Minister Mevlut Cavusoglu later accused Germany of harboring members of the Kurdistan Workers Party (PKK), which has fought an insurgency in southeast Turkey since 1984, and the network of U.S.-based cleric Fethullah Gulen that Ankara blames for a failed coup last July.

“As a country providing shelter to PKK and FETO terrorists in its own territory, statements by Germany are just double standards and unacceptable,” Cavusoglu said on Twitter.