Germany warned 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.

"Everyone can be affected. The most absurd things are possible," said Foreign Minister Sigmar Gabriel in a caution to travelers that highlighted alarm at what Berlin clearly sees as the growing unpredictability of President Tayyip Erdogan.

Gabriel broke off his holiday to return to Berlin and 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.