The deployment of battle groups to the Baltic countries and Poland aimed to send a clear message that "an attack on one NATO ally will trigger a response from the whole alliance," NATO Secretary General Jens Stoltenberg said Tuesday.

In an interview with DW's Teri Schultz at the site of the exercises in central Lithuania, Stoltenberg said the drills were part of a response to "a more assertive Russia."

"We have seen over years a pattern by Russia - more exercises, more investments in military capabilities, and also the use of force against a neighbor in Ukraine," he said.

Moscow annexed the Ukrainian territory of Crimea in March 2014 after Kyiv's pro-Moscow government was ousted. This sent relations between Russia and the West plummeting and led to a separatist insurgency in eastern Ukraine.

"What we saw in Crimea is that there was a lack of strong military presence that made it possible for Russia to act the way it did," Stoltenberg said, adding the German-led multinational battalion in Lithuania aimed to ensure a similar scenario wouldn't unfold in the Baltics. NATO battalions of around 1,000 soldiers each have been deployed in Lithuania, Latvia, Estonia and Poland.

Although Ukraine is not a NATO member, developments in Crimea and the rebel-controlled Luhansk and Donetsk regions were met with concern among Baltic States, some with their own territorial disputes with Moscow.

Stoltenberg said the military alliance had also tripled the size of its response force to 40,000 troops, including a group that could move within days.

No immediate threat

Lithuania's intelligence service has warned that Russia is capable of launching an attack on the Baltics with as little as 24 hours' notice. Moscow has repeatedly denied territorial ambitions in the region, and has criticized NATO's eastward expansion and its military buildup near Russian borders.

The latest drills, set to end on Saturday, involved more than 11,300 troops from 20 NATO countries. One of the exercises focused on defending the Sulwaki Gap - a stretch of the Polish-Lithuanian border sandwiched between Russia's highly militarized Kaliningrad territory and Moscow ally Belarus. Strategists had warned that the gap's capture would essentially cut off the alliance's Baltic members.

Russia is expected to carry out military exercises of its own in September, including a simulated armed conflict with NATO.

Stoltenberg said that while there didn't appear to be any immediate threat against a NATO ally, "we have to be vigilant, we have to follow very closely the developments on our borders."

He also called on Russia to adhere to the Vienna Document, which stipulates rules for international inspection and observation of military exercises. "Russia has not invited international observers before," he said. We "call on them to also be transparent and live up to their international obligations, because incidents and accidents [are] a risk both for us and for them."

nm/msh (AFP, dpa)