The U.S., United Kingdom and Romania each sent troops to NATO ally Poland on Tuesday. The move may have eased concerns over Russia’s proliferation of troops and military equipment near Eastern Europe.

It also appeared as if the fresh deployment came in anticipation of Russia’s reported agreement Monday to send fighter jets, tanks and other armored vehicles to Serbia “soon.”

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Polish Defense Minister Antoni Macierewicz headed a ceremony that welcomed the new deployment in the town of Piotrkow Trybunalski and praised it for aiding against the “threat from the East,” the Associated Press reported.

Russia had increased its military’s activity and deployments along its western border, or NATO members’ eastern border, for months now. It has led to heightened worries from countries like Poland which were quite fretful after President Vladimir Putin’s government annexed Crimea from Ukraine in 2014.

Poland, in particular, as well as other major European powers like France and Germany, had good reason to worry. Satellite images showed Russia appeared to have moved missiles capable of carrying nuclear warheads to the Kaliningrad enclave, just north of Poland, late last year and each was well in range of metropolises like Paris and Berlin.

Perhaps in response, the new NATO troops were made up of four battalions, which consisted of 1,000 troops apiece and were headed to Estonia, Latvia, Lithuania and Estonia. The new influx came on top of the 3,500 troops from a U.S. armored brigade that was sent to Poland in January.

"I would like to thank our allies and commend them on this historic moment, U.S> European commander and Army Gen. Curtis M. Scaparrotti said in a statement more than two months ago. “The European infrastructure and integrated support has enabled our force to rapidly be ready and postured should they need to deter Russian aggression.”

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But Russia seemed undeterred by the NATO troop escalation. Russian Defense Minister Zoran Djordjevic announced Putin had agreed to ship six MiG-29 fighter jets, 30 T-72 tanks and 30 BRDM-2 armored vehicles to Serbia, where they would be “fully modernized and refurbished,” according to the Associated Press.

When and how the vehicles, specifically the jets, would be delivered was unclear. Russia could fly the jets over NATO members’ airspace in order to reach Serbia, or they could be disassembled, shipped on a transport plane and then put back together. However, Russia would need neighboring countries to agree to that.