By Andrius Sytas and David Mardiste
Estonia, Latvia and Lithuania are tightening ID controls and erecting fences on their eastern borders, worried the Baltic region will become a new entry point for refugees as migrant routes through the Balkans becomes harder.
Governments fear they could see thousands of refugees cross from former ruler Russia - the focus of long historical mistrust - and Belarus.
Concerns have grown since around 6000 asylum seekers crossed into Finland and Norway from Russia last year.
Latvia and Estonia have begun to fence off their border with Russia.
Security concerns also played a part after Russia annexed Ukraine's Crimean region and accusations an Estonian security officer was kidnapped on the border region in 2014.
Hundreds of Lithuanian border guards, police and soldiers started an exercise this week over handling a border crisis.
The reintroduction of ID checks on part of the border with its northern neighbour Latvia will also be tested at the exercise.
"Until last year, neither Norway nor Finland had any migration problems on the Russian border," commander of the Lithuanian state border guard Renatas Pozela told Reuters.
"Then migrant flows on that border jumped up in a single week, as if by the wave of a magic wand."
As controls tighten over the direct route from Greece into continental Europe, the route through Moldova, Ukraine and into the Baltic may become more popular, officials say.