The United States has begun to deploy 3,000 troops to the Baltic countries for the three-month exercise Atlantic Resolve, the Pentagon announced on Monday.

Atlantic Resolve will see NATO forces working alongside their allies in Estonia, Latvia and Lithuania.

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 of around 3,000 frontline soldiers.

According to a U.S. 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, U.S. 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."