The Slovak foreign ministry says it is "disturbing" that the Night Wolves - a Russian nationalist biker gang close to President Vladimir Putin - now have a base in Slovakia.
The base has old military vehicles and lies in Dolna Krupa, a village 70km (44 miles) from the capital Bratislava.
The Russian government calls it the Night Wolves' "European headquarters".
The bikers are under US sanctions, accused of providing military help for the pro-Russian rebels in Ukraine.
Slovak foreign ministry spokesman Peter Susko told the BBC that the Night Wolves' activities would have to be "carefully monitored".
"We think the influence of their members is harmful, especially in spreading their opinions that strive to rewrite history," he said in a phone interview.
When asked to specify those controversial opinions he said "that Crimea is, was and will be Russian, that Stalin was a great hero, that Nato is a criminal organisation, etc".
Bikers' Slovak supporters
Slovakia is a member of both Nato and the EU. Formerly part of communist Czechoslovakia, it was an ally of Moscow during the Cold War.
A Slovak nationalist group called NV Europa, led by Jozef Hambalek, is sharing the compound with the Night Wolves.
Mr Hambalek owns the site, which was previously a pig farm, Slovak media report. Last week he threatened Radio Free Europe/Radio Liberty journalists who were filming outside the base.
The Russian bikers say they are turning the site into a World War Two museum honouring Soviet units who used motorbikes.
Speaking on Tuesday, Slovak President Andrej Kiska called the Night Wolves "a tool of the regime that has been involved in the occupation of a neighbouring country" - referring to Russia's intervention in Ukraine.
He called the bikers' base "a serious security risk" for Slovakia.
Mr Susko told the BBC "they claim to be a club" and "they are not a government organisation, so it's difficult to intervene through the [Russian] embassy".
"It's a case of clear concern that people who express views directly contrasting with the foreign policy outlook of Slovakia are trying to organise themselves on the territory of Slovakia," he said.
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So who are the Night Wolves?
Their leader Alexander Zaldostanov - known as "The Surgeon" - has appeared often alongside President Putin. Indeed, in 2011 Mr Putin rode with the Night Wolves at a biker festival in Novorossiysk.
In 2013 Mr Putin pinned a Russian Medal of Honour on Mr Zaldostanov.
The US government accuses the Night Wolves of direct involvement in the Ukraine conflict.
Mr Zaldostanov is among many Russian political and military figures on the sanctions list, accused of involvement in the Russian annexation of Crimea in 2014 and/or the separatists' revolt in eastern Ukraine.
Two businesses linked to the bikers are also on the sanctions list: Wolf Holding of Security Structures, allegedly a supplier of military services, and Bike Center, allegedly run by Mr Zaldostanov.
The US government says the Night Wolves "have been closely connected to the Russian special services, have helped to recruit separatist fighters for Donetsk and Luhansk, Ukraine, and were deployed to the cities of Luhansk and Kharkiv".
It says that during the annexation of Crimea the Night Wolves "participated in the storming of the gas distribution station in Strikolkove and the storming of the Ukrainian Naval Forces Headquarters in Sevastopol".
Slovak TV Noviny reports that the Night Wolves have more than 5,000 members and branches in several European countries, including Serbia, Romania, Macedonia and Bulgaria.
Slovakia is on the bikers' route when they ride across Europe annually to mark the WW2 victory anniversary.
In 2016 Latvia expelled the local Night Wolves chief, Igor Lakatosh, for security reasons. And Poland banned the bikers from entering the country - a move that infuriated the Russian foreign ministry.
What does Mr Zaldostanov say?
In a statement (in Russian) quoted by Mr Hambalek, the Night Wolves leader linked his new base in Slovakia to the Soviet "liberation" of the country in World War Two.
"In Slovakia they pay homage to the heroes who liberated the world from fascism, so the creation of the museum complex, dedicated to the events of World War Two, is logical, legitimate and desirable," Mr Zaldostanov wrote.
"The liberal Western media, gripped by Russophobia and infected with anti-Soviet prejudice, have attacked this historical museum that is being set up painstakingly by enthusiasts - attacked it foaming at the mouth and yelling 'grab it all and share it out'."
He makes frequent trips to Crimea, where he defends Russia's annexation of the peninsula, the population of which is mainly ethnic Russian.