Estonia on Wednesday began a previously unannounced military exercise to erect a barbed-wire barrier on its eastern border with Russia.
Nearly 1,700 reserve soldiers are called upon to participate, according to the government in Tallinn.
The main purpose of the exercise, which will last until November 25, is to test operational readiness and chains of command. Estonia also wants to practise cooperation with police and border guards.
"What is happening in Poland, Lithuania and Latvia requires strengthening the border infrastructure in Estonia as well," said police and border guard chief Elmar Vaher.
The barrier is to be installed at up to 10 border sections where there is increased risk of illegal border crossings.
A build-up of migrants at the European Union's eastern flank has escalated dramatically since last week.
Retaliation for sanctions
The bloc accuses Belarusian strongman Alexander Lukashenko of encouraging migrants to head to the border in retaliation for sanctions imposed by Brussels on his regime.
"The security situation is indeed dangerous and tense," Estonian Prime Minister Kaja Kallas told parliament, referring to clashes between migrants and security forces at the Polish-Belarusian border.
However, there is currently no direct threat to the Estonian border, she said.
Unlike Poland, Latvia and Lithuania, Estonia does not share a border with Belarus.