German convert jihadist (file)
German convert jihadist (file)Screenshot

Germany's federal prosecutor says investigations into suspected fighters with the Islamic State (ISIS) group who have returned to the country have sharply increased within a year, to number nearly 70.

Some 106 returnees suspected of having fought alongside ISIS jihadists are being investigated in Germany in 68 preliminary or criminal inquiries, chief federal prosecutor Harald Range told reporters late Monday, reports AFP.

That compared to five investigations against eight suspects at the beginning of last year, he said, adding that he assumed the number would further increase as authorities seek to clamp down on suspects.

The German cabinet decided in mid-January to tighten a foreign travel ban on known Islamists to stop them heading to war zones such as Syria.

German authorities can already confiscate the passports of known jihadists, but under the new law they will also be able to take away their national identity cards, which can be used for travel to Turkey and within the EU Schengen no-visa area.

Suspects will instead be handed replacement ID cards for periods of up to three years that are stamped with a no-travel advisory in several languages.

The measures come amid concern in Germany and other western countries about the prospect of attacks by returning jihadists.

The Islamic State, which rules a swathe of territory in Syria and Iraq they have deemed an Islamic "caliphate," has attracted thousands of foreign fighters, many from the West.

Police in the southwestern city of Stuttgart on Tuesday carried out searches at a mosque with an education and cultural center suspected of providing support to the ISIS, regional officials said.