Estonia has called up 1,700 reservists for an unannounced military exercise. Part of this is placing barbed wire along the 40-kilometre-long border with Russia.
Unlike the two other Baltic EU countries Latvia and Lithuania, Estonia does not share a border with Belarus.
The three countries warned the European Union last week about a military escalation of the migration crisis in the border region with Belarus.
That country is forcing migrants to cross the border into the EU, increasing the risk of provocations and serious incidents with possible military consequences, the three small countries, which are also members of NATO, wrote.
They have long feared the threat that large neighbour Russia poses to them.