US tanks and fighting vehicles started arriving in Riga on Monday
The shipment coincides with the deployment of a US infantry division and Nato navy drills in the Black Sea.
Although a ceasefire is in place in Ukraine, the conflict has heightened distrust between Russia and the West.
Russia?s foreign minister has accused the EU of stirring up confrontation.
Sergei Lavrov, speaking in Moscow on Tuesday after a meeting with his Spanish counterpart, singled out European Council President Donald Tusk for criticism.
He accused the former Polish prime minister of apologising to President Barack Obama that the EU had not acted as swiftly against Russia as the US.
?EU bureaucrats in Brussels are intentionally escalating confrontation between Russia and the EU,? he said, complaining that they were trying to postpone the political terms of the ceasefire and thereby hinder improvements in relations.
Mr Tusk said in Washington that if the EU and US were united they would be able to ?put a stop to the aggressive policy of Russia against her neighbours?.
Ukraine?s President Petro Poroshenko said on Monday night that Ukrainian government forces had withdrawn the lion?s share of their rocket and heavy artillery systems, while pro-Russian rebels had also ?withdrawn a significant amount?.
However, there is widespread concern among the governments of the Baltic states at Russia?s involvement in eastern Ukraine.
The Kremlin says its forces have not been directly involved in combat, but its denials have been rejected by Ukraine, the West and independent experts.
UK Foreign Secretary Philip Hammond said on Tuesday that President Vladimir Putin?s actions ?fundamentally undermine the security of sovereign nations of Eastern Europe?.
Lithuania is bringing back conscription and President Dalia Grybauskaite told the BBC last week that the threats to her country?s security were ?very real?. She spoke of increased Russian military activity in the Baltic Sea and across the border in the Russian enclave of Kaliningrad.
Russian soldiers also took part in drills in the Pskov region that borders Estonia and Lithuania last month. And last September an Estonian security service official was seized by Russian agents and taken to Moscow.
A US military official told the BBC that some 250 heavy vehicles, out of a total of 750 vehicles, were arriving in the Latvian capital, Riga, and in northern Germany, and would be ?pre-positioned for deployment so we can more quickly react to crises?.
The hardware would be based in the Baltics and Poland for the 90-day Nato training exercise, but would then be spread throughout Europe in ?controlled storage?.
Arrangements still had to be made with host nations, he said.
A Nato naval exercise was also under way in the Black Sea on Tuesday, involving seven members of the Western alliance including Bulgaria, Romania and Turkey.
Russia?s annexation of the Crimean peninsula from Ukraine a year ago has alarmed Nato. Last week Russia began military exercises in several southern areas, including Crimea and the pro-Russian breakaway Georgian region of Abkhazia.