The German government summoned Turkey's ambassador on Wednesday to demand the release of a jailed trainer of human rights activists, and said the foreign minister was interrupting his vacation for consultations on relations with Ankara.

The jailing of Peter Steudtner, who along with Amnesty International's Turkey director and four other activists is accused of aiding an armed terror group, marks a new low point in German-Turkish relations. They have been weighed down by a widening range of issues for months.

Ambassador Ali Kemal Aydin was "told very clearly that the arrest of Peter Steudtner and other human rights activists is neither understandable nor acceptable," Foreign Ministry spokesman Martin Schaefer told reporters in Berlin. The German government demanded Steudtner's immediate release and unhindered consular access, he added.

The accusations of terrorist activity are "clearly far-fetched," Schaefer said.

Foreign Minister Sigmar Gabriel will hold consultations in Berlin Thursday on "what should now be done in view of the dramatic escalation of Turkey's actions," he added.

A massive Turkish government crackdown was initially launched against alleged supporters of last year's failed coup but has since broadened to include government opponents. Nine German citizens are currently in custody.

The activists were detained in a July 5 police raid on a hotel on the island of Buyukada, off Istanbul, where they were attending a digital security workshop. A court jailed the six on Tuesday.

Chancellor Angela Merkel's spokesman, Steffen Seibert, described the terror accusations as "a transparent attempt to discredit and criminalize dissenters."

Earlier Wednesday, a newspaper publisher said it has filed an appeal to Turkey's highest court against the incarceration for the past five months of a German-Turkish reporter.

Deniz Yucel, who works for the daily Die Welt, was arrested Feb. 14 in Istanbul. Turkish authorities accuse him of disseminating terrorist propaganda and inciting hatred, as well as espionage and ties to the outlawed Kurdistan Workers' Party, or PKK— allegations Yucel denies. He hasn't been formally charged.

Die Welt's publisher, WeltN24 GmbH, said it filed a complaint Tuesday to the Constitutional Court in Ankara against Yucel's continued imprisonment and violation of its press freedom in the case.

WeltN24 director Stephanie Caspar said the company "will exhaust all legal means available to us to defend the freedom to report of our correspondent and the publisher."