Around 3,500 USinfo-icon troops have arrived in Poland, one of the largest deployments of US forces in Europe since the end of the Cold Warinfo-icon in 1991, sparking immediate anger from Russiainfo-icon.

The contingent of US troops, which arrived in Poland from Germanyinfo-icon on Thursday along with hundreds of military vehicles, is part of a larger NATO operation to "deter Russian aggression."  The American troops will spend about a month training in Poland before moving to other NATO member states in Eastern Europe for additional training exercises.

The Pentagon operation, dubbed Atlantic Resolve mission, will involve an armored brigade and heavy equipment deployed in Poland and nearby NATO partners Estonia, Latvia, Lithuania, Bulgaria, Hungary and Romaniainfo-icon on a rotational basis.

Outgoing US President Barack Obama ordered the deployment in 2014 to reassure NATO allies in Eastern Europe after Russia's alleged intervention in Ukraineinfo-icon's crisis.

But the mission comes a week ahead of the inauguration of incoming US President Donald Trumpinfo-icon, who has called for closer ties with Russia, despite recent allegations of Moscowinfo-icon's meddling in the US presidential election.

Dmitry Peskov, a spokesman for Russian President Vladimir Putin, said the NATO operation threatens Russia's "interests and our security."

"This is even more pronounced when a third party (the United Statesinfo-icon) reinforces its military presence on our doorstep in Europe," he added.

Alexei Meshkov, Russia's deputy foreign minister, said the deployment is a "factor destabilizing European security."

The first of the arriving troops, reportedly from Fort Carson, Colorado, will be followed with more than 3,000 soldiers, tanks and other military equipment deployed in Poland via Germany and nearby NATO partners.

Among their equipment will be 87 Abrams M1A1 tanks, 20 Paladin artillery vehicles and 136 Bradley fighting vehicles.

The forces will branch out to seven Eastern European countries from Estonia to Bulgaria on a rotational basis within the next two weeks.

In addition to US troops, NATO members Germany, Canadainfo-icon and Britaininfo-icon are also sending up to 1,000 troops, each to the former Soviet republics of Estonia, Latvia and Lithuania.

The US and its allies have been at odds with Moscow since Ukraine's strategic Black Sea peninsula of Crimea rejoined Russia following a referendum in March 2014.

Since then, the European Unioninfo-icon, the US and some other Western countries have imposed several rounds of sanctions against Russia.

NATO also cut ties with Moscow in 2014 and started to deploy troops and weaponry to Baltic States--Latvia, Lithuania and Estonia--near Russia.

Ties between the US and Russia further deteriorated when Moscow last year launched an air offensive against Daesh terrorists in Syriainfo-icon, many of whom were initially trained by the CIA to fight against the Syrian government