BERLIN (AFP) - German neo-Nazis once sported shaved heads, jackboots and olive-green bomber jackets, but security agencies warn that a new breed of far-right extremists has become harder to spot as they promote their ideology of racist hate.

The image of beer-swilling skinheads who assaulted foreigners and torched refugee homes in the 1990s has given way to militants in black-hooded jumpers who organise flashmobs via text messages and social media.

The German domestic intelligence service has been slow to catch up, as highlighted by the case of the National Socialist Underground (NSU) trio who lived in hiding for years and are accused of murdering 10 people, nine with foreign roots. The trial of the group's only surviving member begins Monday in Munich.

The security agency, much criticised for failing to stop the NSU murder spree, estimates that there were 22,400 far-right extremists, including 9,800 who had voiced or shown a willingness to use violence, in late 2011.

They are organised into over 200 clandestine groups with names such as "National Socialist Hatecore" and the international "Blood & Honour" network, said the latest report by the Federal Office for the Protection of the Constitution.

Their members hold nationalist, racist and anti-Semitic views, believe in the superiority of the German people and want to push out immigrants. They also usually promote revisionist views, glorify Adolf Hitler and deny the Holocaust.

Last year, police recorded 842 acts of far-right violence, among more than 17,000 crimes blamed on right-wing extremists.

Many of the offences centre on the illegal use of Nazi symbols and gestures such as the swastika, the insignia of the SS corps, or the right-arm Hitler salute.

Because these are banned, many extremists now wear black "hoodies", baseball caps, scarves and sunglasses, making them difficult to distinguish from their traditional arch-enemies, the left-wing anti-fascist activists.

The murky far-right movement "The Immortals" have donned white face masks similar to those of the hacktivist global "Anonymous" movement in torch-lit night processions through cities, then posted video clips of the events online.

The biggest far-right group that has not been outlawed is the National Democratic Party of Germany (NPD), with some 6,000 members, according to authorities.

Unlike anti-immigrant parties elsewhere in Europe, it has never entered federal parliament, because it has fallen short of the minimum five per cent of the vote. They most recently scored just 1.5 per cent in 2009 national elections.

But the NPD has lawmakers in the state assemblies of Saxony and Mecklenburg-Western Pomerania in what was communist East Germany, economically deprived areas that remain hotspots of far-right extremism.

In some areas here the NPD acts like a legitimate party and is active in youth centres and football clubs. In some towns near the Polish border, it has support of up to 33 per cent.

Germany's state governments this year launched a push to ban the NPD, a decade after a first attempt failed. At the time Germany's highest court argued that the presence of undercover state informants in party ranks had sullied the evidence.

Chancellor Angela Merkel's government has declined to back the latest case, fearing that the legal hurdles to ban any party as a threat to democracy are too high, and that the trial will only generate unwanted publicity for the NPD.