Russia is revamping its former Soviet Union military bunkers along Crimea's southern coast, sites that created a defense barrier against potential naval attacks from the Black Sea in the Cold War era that pitted Moscow against the United States. At least 18 sites are now in use, and they include naval bases, radar stations, airfields and anti-ship missile systems, Reuters reported Tuesday. 

Military vehicles and soldiers in Russian military uniforms have been spotted in recent months inside and near the bases. Crimea borders NATO member Romania, which hosts a U.S.-controlled international missile shield, and could be a strategic territory in any future potential conflicts with the West. 

Russia's military buildup mirrors defense positioning in NATO countries such as Poland, Germany and the Baltic states. Both sides have sent soldiers to the region and held frequent military exercises since Russia's annexation of Crimea from Ukraine in 2014. Most recently, NATO officials held a civil emergency exercise in Montenegro this week while neighboring Serbia hosted 150 Russian paratroops for a Serbian-Russian military exercise.

“We have, in effect, slid into a new Cold War,” Russian Prime Minister Dmitry Medvedev said during a security conference in Munich in February.

Russia already had a significance presence in Crimea before the annexation. It's Black Sea fleet is based in a major naval base in Sevastopol in part because Crimea offers views of the Black Sea, and nearby NATO members Turkey, Bulgaria and Romania.

Britain’s Defense Minister Michael Fallon warned in September about military buildup in Crimea "and indeed the militarization of the Black Sea region generally. Both Bulgaria and Romania feel very threatened."

Moscow has protested Romania's missile shield system as a threat to Russia’s security and vowed to strike back. The NATO system is set to go into affect in 2018.