“The visible change between the two images provided appears to be the fortification of buildings, characteristic of explosive storage bunkers, utilizing earthen berms to further insulate these structures. There also appears to be clearings, new structures visible within the the forested portion of the installation, as well as a berm and exterior fence surrounding the installation,” Hall said in an email.

The unforested sections include explosive ordinance bunkers, he said. “Every structure in the northern non-forested sector have been reinforced during the three month period of the imagery,” he said. “The berms appear to be continually fortified to make them more obscured from aerial detection.”

Berms are mounds of dirt or snow added to fortify structures or restrict access.

“In the forested sector, a different type of storage facility exists, some of which are bermed and appear to be leveled off. There appears to be additional uncovered storage, some with berms but not heavily fortified. In this area some of the structures have changed, potentially showing roofing structures or tarps that have since been removed to reveal caches of items. Some of the berms appear to be more extensive, but the foliage in the second imagery may obscure this analysis. Additionally, there appear to be new or redistributed items — potentially identifiable as shipping containers.”

Hall said the photos also show a railroad line. It presumably connects to the larger national system, which runs to and through Lithuania. Russia built a similar line in the Georgian province of Abkhazia before invading that country in 2008.

Defense One asked NGA officials about recent Russian activity in Baltiysk. They said they could not comment because such information is classified.

In June, a Federation of American Scientists report said that satellite photos of a different site, about eight miles away, show Russia has upgraded nuclear-weapons bunkers on the exclave.

The presence of nuclear weapons in Kaliningrad has been a topic of speculation and concern for years. In 2016, the Russians deployed the SS-26 Iskander, a nuclear-capable missile with a range of more than 400 kilometers, to Kaliningrad. It joins one or more S-400 anti-aircraft batteries, which have a range of nearly 250 kilometers.

Pentagon officials declined to comment on Russian activity at Kaliningrad.

“Without commenting on specific intelligence matters, Russia continues to demonstrate aggressive behavior in Europe,” said Pentagon spokesperson Eric Pahon. “Russia, after the Cold War, retained large numbers of non-strategic nuclear weapons — forces that it is modernizing and increasing as described in the Nuclear Posture Review released in February. Even more troubling has been Russia’s adoption of military strategies and capabilities that rely on nuclear escalation for their success.”

Russia has been adding troops since 2015 to Kaliningrad, which is the home of Russia’s 11 Army Corps and the headquarters of the Russian Baltic Fleet. Baltiysk itself contains a major Russian naval port and the Chernyakhovsk and Donskoy air bases.

Ben Hodges, who retired as the three-star commander of U.S. Army Europe this year, said that the number of Russian troops in Kaliningrad will continue to rise, from 9,000 in 2014 to 14,611 by 2020, in a forthcoming report he co-authored from the Center for European Analysis, or CEPA, obtained by Defense One.

U.S. Ambassador to NATO Kay Bailey Hutchison, in a call with reporters on Thursday, said that Russian aggression would be a key focus of this year’s NATO summit.

“I would say our major areas of deterrence would be Russia and the malign activities of Russia, the efforts of Russia to divide our democratic nation, INF Treaty violations,” Hutchison said. “All of those things are now being addressed by NATO in a strengthened deterrence and defense.”

After the NATO Summit, Trump will meet one-on-one with Putin in Helsinki, Finland, on July 16. During that meeting, “[t]he president will continue to hold Russia accountable for its malign activities,” U.S. Ambassador to Russia Jon Huntsman said on Thursday’s call.

At a political rally in Montana later that evening, Trump downplayed Putin’s attacks on its neighbors and on the West. “You know, Putin is fine,” he said. “He's fine.”

Correction: Due to an editing error, this piece originally misstated which Baltic nation borders Kaliningrad.