(CNN) - Russia may be building up its military in the heart of Europe, evidence provided by an Israeli satellite imagery company suggests.
The images from Imagesat International show large construction work at Russia's base in Kaliningrad, the country’s exclave that sits along the Baltic Sea and borders Lithuania and Poland.
With concern that Russia is upgrading its capabilities at NATO’s doorstep, the U.S. is increasingly on guard.
Cpl. Derek Hussinger was part of a mock invasion force during a recent drill off the coast of Iceland, a U.S. ally, aboard the USS Iwo Jima.
"We're going to land and then need to set up a perimeter and set up battle positions,” he said, explaining the exercise.
It’s one of a number of regular such actions meant to be a deterrent as the North Atlantic region becomes more contested.
With this most recent exercise, the U.S. and its allies are practicing their response if a friendly country were to get attacked.
While the adversary in this exercise was fictitious, it comes at a time of growing tensions between the U.S. and Russia.
Those tension have been exacerbated, as the new evidence from the Israeli company appears to show massive construction work at Russia's bases in Kaliningrad.
It includes what appear to be upgrades to a nuclear storage facility there, an upgraded military airfield, and new, bigger ammunition bunkers.
Russia's defense ministry didn't respond to CNN’s request for information about the buildup.
The commander of U.S. naval forces in Europe and Africa, however, said there's a pattern of Russia upgrading its capabilities in the region.
"They're putting a lot of their modern weapons systems, anti-ship cruise missiles, radars, the bastion system, the S-300 and the S-400, in there,” said Adm. James G. Foggo III, referring to Kaliningrad.
The territory is strategically important for Russia. More than 450 miles from the rest of Russia at its nearest points (similar to how Alaska is disconnected from the rest of the U.S.), it gives Vladimir Putin a foothold in the Baltic and serves as a bulwark against what he sees as NATO’s threatening posture.
Western Europe and the Americans, though, see in a military buildup there the potential for the same kind of intrusions the Russians made into Ukraine against the vulnerable trio of Baltic NATO allies – Lithuania, Latvia and Estonia.
Sending a message of strength to Moscow, the U.S. and its NATO allies are gearing up for an even bigger exercise in Norway.
“If they will challenge us, we will challenge them,” said Adm. Foggo. “We’re not going to be intimidated by those systems that are out there.”