Washington (AFP) - The United States has begun to deploy 3,000 troops on a three-month exercise to reassure Russia's nervous neighbors in the Baltic, military officials said Monday.

Operation Atlantic Resolve will see major NATO forces working alongside their allies in Latvia, Lithuania and Estonia -- former Soviet republics now members of the Western alliance.

Pentagon spokesman Colonel Steven Warren said vehicles, helicopters and heavy equipment had begun arriving in the Latvian capital Riga and that the exercises would last 90 days.

Warren said the deployed unit was a "Brigade Combat Team" -- around 3,000 frontline soldiers.

According to a US military source, speaking on condition of anonymity, the equipment would remain behind in the region after the troops of the Third Infantry Division return to base.

Earlier, in Riga, US General John O'Connor told AFP the deployment would "demonstrate resolve to President (Vladimir) Putin and Russia that collectively we can come together."

The delivery includes Abrams tanks and Bradley fighting vehicles as well as support equipment and O'Connor said the armor would stay "for as long as required to deter Russian aggression."

The three Baltic states of Estonia, Latvia and Lithuania have been NATO and European Union members since 2004 but have very little miliary hardware of their own.

Moscow's annexation of Crimea last year and its meddling in the conflict in eastern Ukraine have galvanized NATO and focused particular attention on its vulnerable Baltic members.

A series of maneuvers by Russia in the Baltic region has stoked concern that the Kremlin could try to destabilize the countries that were in its orbit during Soviet times.

NATO is countering Russia by boosting defenses on Europe's eastern flank with a spearhead force of 5,000 troops and command centers in the Baltic states, Bulgaria, Poland and Romania.