More than a dozen Abrams tanks crossed the Lithuanian border from neighbouring NATO partner Poland on Saturday afternoon, an AFP photographer said.

The deployment, to last until November, is "pre-planned and not associated with any events in the region," a Lithuanian defence ministry statement said.

Lithuania took the lead in European diplomacy on Belarus after veteran strongman Alexander Lukashenko was re-elected in an August 9 vote that the opposition claims was rigged.

Vilnius has given refuge to opposition challenger Svetlana Tikhanovskaya, who claimed victory in the ballot, and has blacklisted Lukashenko along with Baltic neighbours Estonia and Latvia.

Maintaining that he won the ballot fairly, Lukashenko has cracked down on an unprecedented wave of mass protests demanding his ouster.

He has also accused the NATO defence alliance of building up forces in Poland and Lithuania along Belarus's western border.

Warsaw, Vilnius and the Western defence alliance have dubbed these allegations baseless.

Belarus's Soviet-era master Russia, which has long courted Lukashenko as a buffer against the West, has promised him military support.

It is not the first time the US sent a battalion of troops to Lithuania.

During a visit to Vilnius in July, Army Secretary Ryan McCarthy said the United States will continue deploying hundreds of troops for exercises in Lithuania.

Three years ago, NATO deployed permanent troop rotations to Poland and the Baltic states to guard against Russian adventurism on its eastern flank, a region formerly under Moscow's control and spooked by its 2014 annexation of territory from Ukraine.