At the height of the Cold War, the main threat to the allies in Europe was the possibility of Soviet tank divisions crashing through West Germany's "Fulda gap" lowlands to capture the Rhine River bridges and split NATO defenses.

NATO now has another "gap" to worry about, Lt. Gen. Ben Hodges, commander of U.S. Army Europe, said at a Pentagon press briefing Wednesday.

Hodges pointed to the "Suwalki gap," a 60-mile sliver of flat terrain in northern Poland that runs east-to-west from Russian-ally Belarus to Lithuania and the Russian enclave of Kaliningrad, where Russian President Vladimir Putin has placed his most advanced anti-aircraft and anti-ship missiles.

Putin has ratcheted up tensions in the area with frequent, unannounced "snap exercises" of his military near the Suwalki region, combined with the construction of a new airbase in Belarus.

"There's never an observer there" when Putin calls a snap exercise, Hodges said. "We find out about them when they're happening. That's a threat, a concern that we have."

A surprise thrust by the Russians through the Suwalki gap would cut off the Baltic states of Lithuania, Estonia and Latvia -- all NATO members -- and the troops and weaponry already stationed in Kaliningrad could limit a NATO response.

"They could make it very difficult for any of us to get up the Baltic Sea if we needed to in a contingency."

Hodges said he doesn't expect that to happen but he also warned of Putin's unpredictability. He has acknowledged previously that he was surprised by Russia's takeover of Crimea -- and again recently when Putin ordered his warplanes and troops into Syria.

To deter Russia, Hodges has leveraged the 30,000 U.S. troops to rotate through Eastern Europe on constant training exercises while organizing a 5,000-member NATO rapid reaction force.

In another message to Putin, NATO earlier this week invited tiny Montenegro to join the alliance to show resolve against aggression.

Russian Foreign Minister Sergei Lavrov immediately responded: "The most important thing is not to abandon the key principle which forms the basis of our relations with NATO, which is equal and transparent security, when no-one can strengthen its security at the expense of other states," he said.

Secretary of State John Kerry said that NATO was a defensive alliance and the invitation to Montenegro was "meant to simply provide security and I don't think it's focused on Russia per se, or anybody else."

On Ukraine, Hodges said that Russia's commitment of major assets to Syria has not diminished Moscow's ability to support the separatists in eastern Ukraine. "They definitely have the ability to do both, should they need to," Hodges said.

--Richard Sisk can be reached at Richard.Sisk@military.com.