WASHINGTON: US President Joe Biden on Sunday backed the right of Amazon workers to unionize, but stopped short of explicitly encouraging them to form a union.

Workers at an Amazon warehouse in Alabama began voting in February on whether to form the first American union at the e-commerce giant that could pave the way for further unionization in the US at one of the world's most powerful companies.

"Workers in Alabama and all across America are voting on whether to organize a union in their workplace," Biden said in a video posted to Twitter. "This is vitally important -- a vitally important choice."

"There should be no intimidation, no coercion, no threats, no anti-union propaganda," he said. "Every worker should have a free and fair choice to join a union."

Amazon does not officially oppose unions but it has campaigned against efforts by staff at the warehouse in Bessemer to unionize.

In addition to flyers posted in bathrooms, it has held meetings to make the case against unions and even launched a website arguing that a union is unnecessary.

There have been a series of protests around the United States on safety and working conditions at Amazon, with the pandemic increasing pressure on its distribution network even as profits soar.

The company -- helmed by the world's richest person, Jeff Bezos -- has maintained that it has invested billions in worker safety even as it has boosted the number of its employees.

It is unclear how many of the Bessemer employees support a union, with the count expected to begin on March 30.

If a majority vote in favor, they will automatically be represented by the Retail, Wholesale and Department Store Union (RWDSU) but will only become full members after approval of a new company contract, negotiated between the union and Amazon.

"As President Biden points out, the best way for working people to protect themselves and their families is by organizing into unions," RWDSU head Stuart Appelbaum said in a statement.

Biden cultivated a pro-organized labor image and played up his working class background while campaigning for office, although he had previously avoided addressing the Bessemer efforts to unionize.