BERLIN (REUTERS) - Police have arrested two German men suspected of being members of insurgent group Islamic State in Iraq and Syria (ISIS), the federal public prosecutors office said on Thursday.

Prosecutors said, however, there was no evidence pointing to concrete plans or preparations for them to carry out an attack.

The two men, identified by the prosecutors office in Karlsruhe only as 26-year old Mustafa C. and 27-year old Sebastian B., were arrested in North Rhine-Westphalia state.

Authorities are worried about possible attacks in Germany by Islamists who have returned to Germany after joining Islamic State and other similar groups in Syria and Iraq. Fears have heightened after this month's Islamist attacks in France.

Security services say about 600 German residents have joined militant groups in Syria and Iraq. Of those, about 60 have been killed and around 180 are thought to have returned home.

The two suspects are accused of travelling to Syria via Turkey in 2013 and joining a fighting group "Muhajirun Halab"(Aleppo Migrants) which belonged to a group that later joined Islamic State.

They are also suspected of having trained to fight a holy war and to have undertaken logistical tasks such as transport for food and supplies to the front line there.

Mustafa C. is also accused of being responsible for propaganda within his fighting group. Sebastian B. returned to Germany in November 2013 and Mustafa C. Returned from Syria in September last year.