"NATO today officially activated NATO force integration units in Lithuania, Latvia, Estonia, Poland, Bulgaria and Romania," ministry spokeswoman Asta Galdikaite told AFP.
The Baltic nation's foreign minister Linas Linkevicius said the move was "a clear sign that NATO takes it's commitments very seriously".
"It is a clear message both to allies and, of course, opponents who have recently increased the tensions," the minister told AFP, without directly naming Russia.
NATO Secretary General Jens Stoltenberg will attend ceremonies in Vilnius Thursday formally inaugurating the string of units, an alliance statement said.
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It is the latest NATO move to boost defences in the eastern flank since Russia annexed the Crimean peninsula from Ukraine last year.
NATO has increased the number of military exercises, deployed planes and ships while the US pledged to keep heavy weapons in the region that lay behind the Iron Curtain a quarter of a century ago.
Latvia confirmed Tuesday that two US Army Predator surveillance drones and 70 airmen had deployed to its Lielvarde Air Base for a training mission.
Intelligence training will involve two intelligence officers from Poland and each of the Baltic nations of Latvia, Lithuania, and Estonia.
The moves come less than two weeks after NATO opened a new Strategic Communications Centre of Excellence in the Latvian capital, Riga.
Though officials have been at pains not to describe it as a counter-propaganda centre, part of its brief is to analyse aggressive messages emanating from media operated by Russia.
The three Baltic states and Poland have also repeatedly called for a permanent NATO presence to deter Russia but the Western alliance has so far stuck to back-to-back troop rotations.