Security officials in Germany have visited known far-right extremists to warn them not to travel to fight in Ukraine.
It is one method western states are using to discourage extremists from taking up arms amid fears returning right-wing fighters will pose a security threat.
In a webinar hosted by the Counter Extremism Project, experts warned European nations to prepare for the fallout.
“If we have groups of highly motivated, combat experienced far-right extremists with post-traumatic stress disorders, then attacks in European countries may look very different in the future,” terrorism expert Alexander Ritzmann said.
“We need to prepare societies so we have measures in place ready to deal with the returnees.”
He said CEP was working with officials in Germany to identify violent extremists.
“The federal government is reaching out to far-right extremists telling them not to go” to Ukraine, he said.
On Wednesday, German authorities investigating neo-Nazis arrested four people they said were leading members of a far-right combat sports group.
They said the organisation, Knockout 51, held street-fighting sessions to indoctrinate young men and has links to other far-right extremists in Germany.
The group tried to set up a “Nazi neighbourhood” in Eisenach and its members injured several people, prosecutors said.
In all, 61 properties in various parts of Germany were raided.
Prosecutors said they were investigating 21 people suspected of keeping alive the Combat 18 Deutschland group, despite a ban by German authorities, and 10 people suspected of involvement in the Atomwaffen Division Deutschland terrorist group.
CEP director Hans-Jakob Schindler said that far-right extremists would still travel to Ukraine and those who survived would return.
He said good lessons need to be learnt from the way nations have dealt with returning ISIS fighters.
“It is difficult to manage the risk when it is not clear who has and has not gone,” he said.
“But we have counterparts in Ukraine that the authorities can engage with to compile data on individuals.
“It should also be possible to get lists from Ukraine on who has registered as volunteers and for them to work out who are violent extremists. There needs to be co-operation with transit countries. They can act as early-warning mechanisms on who is returning.”