Amnesty International's Turkey chief and 10 other human rights activists in the country have been charged with allegedly belonging to and aiding terror groups.

Police detained 10 activists at a hotel near Istanbul in July, arresting eight people including Amnesty International's director in Turkey Idil Eser, as well as German Peter Steudtner and Swede Ali Gharavi. Two others were detained but released pending trial.

They are accused of aiding armed terror organisations in alleged communications with suspects linked to Kurdish and left-wing militants as well as the movement led by US-based Muslim cleric Fethullah Gulen.

Mr Gulen is accused of orchestrating last year's coup attempt but denied the accusations.

The case of Amnesty's chairman Taner Kilic, a lawyer imprisoned in June, was added to the indictment. He was accused of using an encrypted mobile messaging application, allegedly used by Mr Gulen's network.

The arrest of Mr Steudtner and other Germans has caused serious friction between Berlin and Ankara, and German foreign minister Sigmar Gabriel said the announcement of charges was a matter of great concern.

He called the accusations against Mr Steudtner absolutely incomprehensible and the threat of 15 years in prison for him unacceptable, saying his office immediately contacted the Turkish government.

"We continue to do everything we can to bring the imprisoned German citizens, including Peter Steudtner, back to Germany," said Mr Gabriel.

The 17-page indictment, prepared by the Istanbul prosecutor's office's terror crimes branch, reportedly includes a "secret witness" who claimed the activists were engaged in illegal activities.