German police on Thursday raided sites across the country linked to a far-right group banned by the interior ministry, weeks after a suspected extremist gunman shot dead nine people of migrant backgrounds.

"Since the early hours, police measures are going on in ten states" out of Germany's 16, interior ministry spokesman Steve Alter wrote on Twitter.

"For the first time, the interior minister has banned a 'Reichsbuerger' (Citizens of the Reich) group," Alter added.

"Even in these times of crisis, we will fight far-right extremism, racism and anti-Semitism."

The "United German Peoples and Tribes" organisation banned Thursday belongs to a wider "Citizens of the Reich" movement fed by conspiracy theories.

Its adherents question the legitimacy of the modern Federal Republic of Germany and have in the past entered into armed confrontations with police.

In a 2016 shootout, a Reichsbuerger-linked man killed an officer and wounded two more.

He was later sentenced to life in prison.

After a racist gunman shot dead nine people of migrant backgrounds in the city of Hanau last month, Interior Minister Horst Seehofer declared far-right extremism "the biggest security threat facing Germany" and announced increased police measures.

Seehofer said the far right had left "a trail of blood" in recent months -- two people died in an attack on a synagogue in the city of Halle in October and a pro-migrant politician was murdered at his home in June.

Separately, 12 men were arrested across Germany in February on suspicion of planning attacks on mosques aimed at bringing about "a civil-war-like situation".

The government has announced hundreds of new posts for federal police and security services to strengthen surveillance of the far-right scene, and is considering tighter laws on gun ownership.