Satellite images indicate that Russia is upgrading several military installations in Kaliningrad, a disconnected piece of Russian territory serving as a strategic military outpost on NATO's doorstep.

Reports of a possible Russian military buildup there come ahead of a massive NATO exercise involving tens of thousands of troops.

" If they want to challenge us, we will challenge them," Adm. James Foggo III, commander of US Naval Forces Europe-Africa and the Allied Joint Force Command, told CNN, the outlet that exclusively obtained the satellite images of activities in Kaliningrad.

Russia is upgrading and modernizing four military installations in a strategic area on NATO's doorstep, satellite images obtained by CNN suggest.

Increased Russian military activity has been spotted in Kaliningrad, a disconnected Russian territory situated between Poland and the Baltic states. The Russians have been carrying out major renovation work at what is believed to be an active nuclear weapons storage site, Hans M. Kristensen with the Federation of American Scientists concluded in an analysis of satellite images in June.

Russian operations in the area appear to have dramatically expanded in recent months.

Work continues at the apparent nuclear storage site identified earlier, and another 40 new bunkers, each with the potential to serve as military storage facilities, are under construction near Primorsk, a large port on the Baltic Sea. Upgrades to the Chkalovsk air base, including a new railway and improved aircraft landing system, and the Chernyakhovsk base, home to a Russian missile brigade, are underway, CNN reported Wednesday.

The nuclear-capable Iskander missile was delivered to the Chernyakhovsk base in February. This troubling delivery is recognized as one of the more serious signs of Russian militarization in the Baltics.

"If they want to challenge us, we will challenge them," Adm. James G. Foggo III, the commander of US Naval Forces Europe-Africa and the Allied Joint Force Command in Naples, Italy, told CNN without specifically commenting on the satellite images. "We're not going to be intimidated by those systems that are out there."

Reports of a possible Russian military buildup in Kaliningrad come just ahead of a massive NATO military exercise involving tens of thousands of troops from more than two dozen countries.

The upcoming Trident Juncture exercises, scheduled to begin next week, will include 45,000 troops, 10,000 vehicles, 60 ships, and 150 aircraft from 31 countries training side by side in and around Norway. The joint drills, Article 5 collective defense exercises, will include land, air, and amphibious forces training to repel an adversary threatening the sovereignty of a NATO ally or partner state.

The Russians have been invited to observe the exercises, which are designed to send a message to Moscow.

"There's a strong deterrent message here that will be sent," Foggo explained to press at the Pentagon earlier this month. "They are going to see that we are very good at what we do, and that will have a deterrent effect on any country that might want to cross those borders, but especially for one nation in particular."

Tensions are running high between Russia and NATO, and Kaliningrad is a potential fault line for regional conflict.