In what could mark an escalation of tensions with the West, commercial satellite images suggest that Russia is moving a new generation of nuclear-capable missiles into Eastern Europe.

Russia appears to be preparing to permanently base its Iskander missile system in Kaliningrad, a sliver of territory it controls along the Baltic coast between Lithuania and Poland. Arms control experts shared fresh satellite imagery with NPR, which they say provides evidence that the Iskander will soon be housed in the Russian-controlled enclave.

The images show ground being cleared for tent-like shelters used at other Iskander bases, says Jeffrey Lewis, a nonproliferation expert at theJames Martin Center for Nonproliferation Studies. "The pattern, and the size, and the location strongly suggest to us that this is the beginning phase of construction of the shelters for Iskander," Lewis says.

Lewis and Finnish defense analyst Veli-Pekka Kivimäki discovered the construction through digital sleuthing. First, they searched Russia's Facebook, known as VKontakte, for images taken by military conscripts assigned to missile units (Russian grunts are prolific on social media, according to Lewis). Comparing the images posted by conscripts to the satellite imagery, they were able to pinpoint the missile base in Kaliningrad where the Iskanders have sometimes been sent on training exercises.

They then monitored the bases until they saw construction of what they recognized as permanent storage structures used for Iskander missiles.

Lewis says placing Iskander missiles in Kaliningrad is a provocative act. Kaliningrad has been controlled by Russia since World War II. It lies far to the west of Russia's own border, putting any missiles based there within range of additional targets in Europe. "Things that are in Kaliningrad... can reach places that they could not otherwise reach in Russia," Lewis says.

Lewis and other experts believe Russia may be also developing a longer-range cruise missile that would allow the Iskander system to reach targets in Western Europe as well. That missile, if it does exist, would violate the 1987 Intermediate-Range Nuclear Forces Treaty, which prohibits medium-range cruise missiles from being deployed on the ground.

If Russia has decided to permanently position Iskander in Kaliningrad, "It may be in response to a number of things," says retired Brig. Gen. Peter Zwack, who served as defense attaché to Russia from 2012 to 2014.

The U.S. has recently deployed a missile defense system in Romania and is building a second base in Poland. U.S. and European officials say those sites are to defend against potential ballistic launches from Iran, but Zwack says that Russia views them as provocations. The new Polish missile defense site would be within range of the Iskander, adds Lewis.

But Zwack says it's important not to overreact. Russia already has nuclear-capable systems based in Kaliningrad, including SS-21 ballistic missiles. The new Iskanders will "freak the local neighbors," he says, but they "will not change any strategic equation, because if they go into tactical mode, it's the end of the world anyway."

Ultimately, Zwack says he believes any decision to put Iskanders into Kaliningrad is about sending a message to NATO and the West that Russia disapproves of their activities.

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