The executive council of the Kurdistan Workers Party (PKK) called on Chancellor Angela Merkel's government to give up "its hostile policy towards the Kurdish people and its liberation movement" or take responsibility for "the negative consequences", the magazine said, without naming its sources.

The demand was transmitted via the German embassy in Turkey at the end of June.

Shortly afterwards Turkish police informed their German colleagues of heightened activity within Kurdish circles in Turkey and warned of the likely risk of kidnappings or attacks.

The German police passed on the information to the interior ministries of several countries, according to Der Spiegel.

Secretary of state for the interior August Hanning nevertheless acknowledged to the magazine that, "We will probably also have to prepare ourselves for new dangers on German soil."

On Thursday, German Foreign Minister Frank-Walter Steinmeier rejected demands for a change of policy towards the PKK in exchange for the freedom of the three German climbers snatched on Mount Ararat in eastern Turkey.

The PKK has waged a bloody struggle for self-rule in Turkey's Kurdish-populated southeast since 1984.

It is listed as a terrorist group by Ankara, the European Union and the United States and has been banned in Germany for 15 years.

The group has kidnapped people in the past, among them soldiers, police and tourists, but this is not a tactic it frequently employs.