Czech and Slovenian authorities have voiced alarm over the emergence of armed anti-migrant militias in the two central European countries.

The concerns come after revelations of a paramilitary base, with tanks and armoured personnel carriers, used by a biker gang with Kremlin ties in Slovakia.

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The Czech intelligence service, the BIS, voiced its worries about a group that calls itself the National Home Guard in a classified report seen by Czech daily Mlada Fronta Dnes.

"Parts of this group have begun to adopt the concept of armed groups. Due to the fact that some of the members are strongly xenophobic, racist, and completely reject the orientations of Czech internal and foreign policy, they could pose a significant [security] risk," the BIS report said.

The home guard groups, which have up to 2,500 members in 90 national branches, patrol the streets of some small Czech towns, such as Nymburk, 50km west of Prague, looking for irregular migrants.

They appear to have links with local police and have political support from National Democracy, a fringe far-right party.

They also have ideological leaders, such as David Buchtel, a Czech academic and National Democracy member, who publishes leaflets saying that Nato plans to "occupy" the Czech Republic and force it to take in migrants.

The Czech foreign ministry has said the groups posed a risk of violent protests, such as in the recent anti-migrant riots in the town of Chemnitz, Germany.

Andor Sandor, the former chief of Czech military intelligence, the VZ, told Radio Prague, that even if it does not come to that, their day-to-day activities posed a threat to the Czech political landscape.

"This could stem from the view, that the European Union is not able to manage the migration crisis. People who believe that neither the state nor Europe can manage this [migration], will take matters into their own hands to protect their families and their property," he said.

The Czech worries surfaced a few days after a social media stunt by Andrej Sisko, a far-right politician, which caused alarm in Slovenia.

Sisko posted a Facebook video of himself with a group of some 70 masked men armed with machine guns in the Slovenian countryside.

The group, called the Stajerska Guard, was filmed taking an oath to secure public order in the country. It numbers several hundred people in total, the Reuters news agency said.

"We are doing nothing wrong and we would be even interested in co-operating with the police," Sisko said, in an echo of the Czech home guard's modus operandi.

His political party, the anti-migrant United Slovenia Movement, has also vowed to protect the county's ethnic identity.

Borut Pahor, the Slovenian president, said: "Slovenia is a safe country in which no unauthorised person needs or is allowed to ... illegally care for the security of the country and its borders".

The creation of the Stajerska Guard was "absolutely unacceptable" and it "needlessly stirs up fear and spreads hatred", outgoing Slovenian prime minister Miro Cerar said.

Earlier in July, Slovakia was also put an alert when journalists filmed a paramilitary compound in Dolna Krupa, a town some 50km north of Bratislava.

The base, a former pig farm, is used by the Night Wolves, a biker gang, and by two far-right militias called the Slovak Levies and NV Europa, the BBC reported at the time.

It contained a shooting range and tanks and armoured personnel carriers that had been supplied by a military vehicle museum.

The revelations were "disturbing" and the groups' influence was "harmful, especially in spreading their opinions that strive to rewrite history", a Slovak foreign ministry spokesman said.

The Night Wolves gang has well known links to the Kremlin.

The other paramilitary groups and their political supporters also repeat Russian propaganda lines on migrants and EU failures, but neither the Czech or Slovene authorities spoke of Russian involvement in their activities.

The notion of a 'migrant invasion' in central Europe is not borne out by facts.

The Czech Republic took in 12 migrants from Greece and Italy under an EU scheme and granted asylum to just 145 people last year.

Slovenia granted asylum to 152 people last year.

Slovakia has boycotted the EU scheme, along with Hungary and Poland, and had just 56 applications for asylum as of June this year.

But the Czech intelligence assessment that the home guard group "completely [rejected] the orientations of Czech internal and foreign policy," was also open to question.

Czech prime minister Andrej Babis has vowed to join an anti-migrant political axis in Europe alongside Hungary and Italy's far-right leaders.

Meanwhile, anti-migrant rhetoric by leading politicians has become a mainstay in Slovakia and Slovenia, where the far-right Slovenian Democratic Party became the biggest one in June elections, but failed to find coalition partners to form a government.