Polish tanks participate in a military exercise in Swietoszow, western Poland, on June 13. (Lech Muszynski/European Pressphoto Agency)

Russia would swiftly respond to any moves by the United States to build up military resources in Eastern Europe by fortifying its western border with more troops, tanks, planes and missile systems, a defense official told the Russian press Monday.

The Pentagon is considering plans to store heavy weaponry, tanks and other vehicles in the Baltics, U.S. officials said Saturday . If that happens, Russia would view the move as “the most aggressive step since the Cold War,” Russian army Gen. Yury Yakubov told the Russian news service Interfax.

Yakubov said Russian forces “along the entire perimeter of Russia’s western border will be reinforced” as soon as Russia notes the buildup of any American heavy military equipment in the Baltics or Eastern Europe.

U.S. officials said the proposal, if approved, would put extra weapons and vehicles in countries that might include Lithuania, Latvia, Estonia, Poland, Romania, Bulgaria and Hungary.

In that case, “Russia won’t have anything else to do but bolster its forces and resources on the western strategic theater of operations,” Yakubov said.

Plans to fortify military resources in Eastern Europe have yet to get the green light from Defense Secretary Ashton B. Carter, U.S. officials said.

Such an approval would be historic: The United States has not sent heavy weapons to the NATO states that were once Soviet republics since they gained independence at the close of the Cold War. The Baltics joined NATO in 2004, along with former Iron Curtain satellite states Romania and Bulgaria, while Poland and Hungary became member states in 1999.

Some of those governments have been urging NATO to build up its resources in the area in light of Russia’s recent annexation of Crimea and its support for pro-Russian separatists fighting government troops in eastern Ukraine. While Russia has made no overt attempt to seize territory in a NATO member state, Poland and the Baltic states have argued that more deterrence is necessary and have called on NATO to focus its missile shield at Russia.

Col. Steve Warren, a Pentagon spokesman, said Monday that officials had tried to position equipment closer to training sites in Europe for some time, meaning that equipment did not need to be moved long distances when training occurs.

“It just makes it more efficient; it saves taxpayer dollars,” Warren told reporters. He disputed the notion that a possible decision to place equipment or weaponry in Eastern European or the Baltic states was linked to the conflict in Ukraine.

“This is purely positioning of equipment to better facilitate our ability to conduct training,” Warren said.

NATO forces have also been stepping up military exercises in Eastern Europe and the Baltics in recent months, a period during which Russia has also staged more military exercises.

But NATO has not deployed permanent combat forces or stores of heavy equipment in Eastern Europe since it pledged not to in a 1997 agreement with Russia about collective defense and security. U.S. officials have also repeatedly said — most recently last month, when a top Ukrainian security official said Ukraine was interested in building missile defenses to protect against Russia — that the alliance’s most serious weapons systems are not focused against Russia.

In an interview with the Italian newspaper Corriere Della Sera this month, Russian President Vladi­mir Putin said “only an insane person” would think Russia would attack NATO.

But it appears that Russia has at least basic plans in place to answer a potential buildup of NATO reinforcements.

Yakubov told Interfax that Russia would answer such a move by outfitting its missile brigade in the European region of Kaliningrad “with new Iskander tactical missile systems” and making changes to its defense interests in Belarus.

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