More than 900 police officers raided homes and clubhouses of suspected neo-Nazis in a crackdown in a western German state on Thursday, seizing far-right propaganda material, computer hard drives and a wide variety of weapons, the region's top security official said.

The raids to collect evidence came after North Rhine-Westphalia state interior minister Ralf Jaeger banned three local neo-Nazi groups.

"These groups are anti-foreigner, they are racist and they are anti-Semitic," Jaeger said at a press conference.

The crackdown in North Rhine-Westphalia comes amid a greater focus nationally on the far right in Germany. That was sparked by the revelation last year that a small group of neo-Nazis apparently managed to kill nine minorities and a police officer over a seven-year period while remaining off the radar of the country's intelligence services.

The existence of the National Socialist Underground group only came to light last November, when two of its three core members were found dead in an apparent murder-suicide after a failed bank robbery. The third alleged core member, Beate Zschaepe, is in custody pending trial.

In a report released last month by Germany's domestic intelligence agency, officials estimated that the wider membership of neo-Nazi groups in Germany fell to 22,400 last year from 25,000 in 2010, but the number of far-right extremists prepared to use violence grew to 9,800 from 9,500.

While a lot of the attention has been focused on states in former East Germany, where the far-right National Democratic Party holds seats in two parliaments, the raids on Thursday focused on the western cities of Dortmund, Aachen and Hamm.

In total, authorities raided 146 buildings in 32 North Rhine-Westphalia cities and towns, Jaeger said.