NATO chief Jens Stoltenberg has said he sees no "imminent threat" from Russia's upcoming military maneuvers with Belarus, but criticized Moscow for not being more open about the drills.
The Zapad (West) 2017 exercise, which Moscow says will involve some 12,700 troops, has caused concern in Poland and the Baltic states.
Lithuania and Estonia say that as many as 100,000 soldiers could take part, though Russia insists the event is "purely defensive" in nature.
NATO has deployed four battle groups -- around 4,000 troops -- to Estonia, Lithuania, Latvia, and Poland in recent years in response to growing Russian assertiveness in the region, particularly after it annexed Crimea from Ukraine in 2014.
Stoltenberg, speaking during a visit to a NATO contingent in Tapa, Estonia, said the alliance's troop presence sent a clear message "that an attack on one ally will trigger a response from the whole alliance."
"We will monitor the [Zapad] activity closely and we are vigilant but also calm, because we don't see any imminent threat against any NATO ally," he said.
But Stoltenberg said Russia's openness about Zapad-2017 did not meet international standards.
Three NATO experts have been invited to attend as observers, but Stoltenberg said this "fell short of the transparency required by the OSCE," the pan-European security body.
"Transparency and predictability are even more important when tensions are high, to reduce the risks of misunderstandings and incidents," Stoltenberg said.