Istanbul (AFP) - Turkish President Recep Tayyip Erdogan on Friday told Germany it cannot scare Ankara with threats, in an escalating row over a wave of arrests that prompted Berlin to step up warnings to German tourists and investors.

"They (Germany) cannot scare us with these threats, they should know this," Erdogan said in a speech in Istanbul. "You (Germany) do not have the power to smear Turkey... or the power to scare us," he added.

Relations between key NATO partners Turkey and Germany -- already brittle in recent years -- have been further eroded in recent days by an intensifying row over the Turkish authorities' recent wave of arrests.

Responding to German criticism over the rights situation in Turkey, Erdogan declared that Turkish courts were "more independent" than German ones.

"Germany should sort itself out," he added. He said Germany had to account for the "terrorists" who Erdogan said were hiding in Germany.

Turkey has repeatedly accused Germany of harbouring both Kurdish militants and suspects wanted over the July 15, 2016 failed coup.

The German foreign ministry had warned it could no longer guarantee its citizens' safety in Turkey due to the current series of arrests which have included German nationals.

But Erdogan described the warning as "baseless and malicious".

He also denied claims that Ankara was probing dozens of top German firms on suspicion of supporting terror groups as "black propaganda".

"You have no power to darken Turkey," Erdogan said.

The latest crisis was precipitated by the order of a Turkish court to remand in custody six human rights activists detained on an island off Istanbul including Amnesty International's Turkey director Idil Eser and Berlin-based activist Peter Steudtner.