BERLIN (Reuters) - German police have raided the home of two Syrian brothers with links to the militant group Islamic State (IS), suspecting that they were preparing an attack, prosecutors in Frankfurt said on Thursday.

Police confiscated an air pistol, electronic storage devices, mobile phones and 14,000 euros ($16,000) in cash but did not arrest the brothers, 21 and 30 years old.

Prosecutors did not give more information on the exact nature of the suspected crime, which they called a "serious act of violent subversion".

The older brother had entered Germany in February 2015 with a forged passport obtained by IS, a crime for which he was sentenced and fined last year.

Prosecutors said that in social media postings he had promoted the militant group, threatened German authorities and justified last November's attacks in Paris.

The younger brother published a picture of himself on social media that showed him sitting in a "luxury car belonging to his brother" sporting a pistol, the prosecutor's office said.

Germany has been on alert since militants with links to Islamic State killed 130 people in Paris in November.

The anxieties have been fueled by the arrival of over 1 million migrants in Germany last year, many of them fleeing war and conflict in the Middle East and beyond.

Last month, the government said that the whereabouts of more than 140,000 people registered in 2015 were unknown. ($1 = 0.8839 euros)

(Reporting by Tina Bellon)