In a move to help reassure NATO allies facing an unpredictable Russian threat, the U.S. will deploy about 250 tanks, artillery and other military equipment in six former Soviet bloc nations.

Defense Secretary Ash Carter announced the plan Tuesday in Estonia about 125 miles from the Russian border as he stood with defense chiefs from Estonia, Latvia and Lithuania, reported The Associated Press.

"We didn't want to have this new challenge," Carter told U.S. marines and sailors on the San Antonio warship that had just completed drills in the Baltic Sea, reported Reuters. "But then all of the sudden here you have behavior by Russia, which ... is an effort to take the world backward in time. And we can't allow that to happen."

The U.S. and NATO are "committed to defending the territorial integrity of Estonia, Latvia and Lithuania," Carter said.

Most of the military equipment will reside in Estonia, Latvia, Lithuania, Bulgaria, Romania and Poland, and some will also be sent to Germany, Carter said.

He told reporters that the equipment will be moved around depending on which country the military exercises are being held in. "They're not static. Their purpose is to enable richer training and more mobility to forces in Europe," he said, according to AP.

Carter also announced plans to work with NATO's cyber center in Estonia to help allies develop better cyberwarfare strategies.

The announcement comes just a day after he revealed that the first part of the two-pronged plan - the placement of other weapons, aircraft and special force outfits in Europe, which will remain on standby as part of NATO's new rapid reaction force.

Estonia Defense Minister Sven Mikser said he doesn't intend to provoke Russia into a new Cold War arms race or challenge Russian President Vladimir Putin "tank for tank," but he said the extra military presence will act to deter any potential Russian aggression, according to AP.

"In global terms Russia is no match conventionally to U.S. or to NATO, but here in our corner of the world, Putin believes that he enjoys regional superiority," Mikser said, reports Reuters. "We have reasons to believe that Russia views the Baltic region as one of NATO's most vulnerable areas, a place where NATO's resolve and commitment could be tested."

The increased military presence is largely in response to the Russian annexation of Crimea and its perceived military involvement with pro-Russian rebels fighting against the Ukrainian government in eastern Ukraine.

U.S. officials said Russia's involvement in Ukraine highlights the importance of an agile response force capable of countering Russian "hybrid warfare" that involves a mixture of unidentified troops, propaganda and economic pressure, according to Reuters.

Putin has adamantly denied any involvement in the area, instead accusing the West of stirring tensions. He announced earlier this month that Russia will add more than 40 new intercontinental ballistic missiles to its nuclear arsenal this year, which will be "able to overcome even the most technically advanced anti-missile defense system," RT reported.