ZAGAN, Poland (Reuters) - The United States deployed thousands of soldiers and heavy weaponry to Poland, the Baltic states and southeastern Europe on Monday in its biggest buildup since the Cold War, condemned by Russia as part of an aggressive strategy on its frontiers.

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The deployment of a full armored brigade, returning U.S. tanks to Europe for the first time since 2013, offers reassurance of U.S. support for east European allies alarmed by Moscow’s military assertiveness towards Ukraine, in the south, and by Russian air, land and sea exercises in the Baltic region.

Originally ordered by Barack Obama, the troop arrivals are the first since Donald Trump succeeded him and may go some way to assuage the alarm of 27 allies over his recent description - since modified - of the alliance as “obsolete”. His expressed wish to move closer to Russia has also caused dismay.

Polish Defence Minister Antoni Macierewicz declared “God bless President Trump” at an opening ceremony with U.S. generals who watched live tank fire at a base near the German border.

“The full weight of the U.S. military stands behind these sentinels of freedom,” Lieutenant General Tim Ray, deputy commander of U.S. European Command, said of the dozens of U.S. soldiers and tanks lined up in the snow, some of which were still painted in desert colors of the Middle East.

WIDER NATO BUILDUP

Concern has grown especially in the three formerly Soviet Baltic republics - Lithuania, Estonia and Latvia - since Russia’s annexation of Ukraine in 2014 and the raising of a pro-Russian rebellion in eastern Ukraine that continues to simmer.

The presence of 3,500 U.S. soldiers, who will be joined by another 1,600 as well as helicopters and aircraft next month, is part of a wider, multinational build-up of NATO nations.

U.S. soldiers will also deploy to Bulgaria and Romania.

Lieutenant General Ben Hodges, the U.S. army’s top commander in Europe, said the build-up also underlined reassurance Trump gave to the leaders of Britain, Germany and France in recent days over the importance of NATO.

“I don’t see, and I have not heard, anything that would lead me to think that this could be curtailed,” Hodges said standing in front of dozens of tanks lined up in the snow.

“Our president has spoken with Prime Minister (Theresa) May, Chancellor (Angela) Merkel and President (Francois) Hollande and what I have heard is the affirmation of the importance of NATO and the United States commitment to NATO,” Hodges said of Trump’s phone calls and May’s trip to Washington.

Britain, Canada and Germany are also sending troops to the Baltic States.

“We will deter and defend across the whole spectrum of conflict,” U.S. ambassador to Poland Paul Jones said, pledging that the United States, Canada and European allies will “defend every inch of alliance territory together.”