Billionaire former New York City Mayor Michael Bloomberg has filed paperwork for the Democratic primary election in Alabama, paving way for US presidential bid.

Bloomberg, 77, who has not formally confirmed his candidacy, submitted on Friday the documents to meet Alabama's filing deadline. The move is a necessary step to join the race to be the party's candidate in next year's election.

The Democratic field, now numbering 17 candidates has coalesced into four top contenders according to recent polls: US Senators Elizabeth Warren and Bernie Sanders, who represent the party's progressive wing, and former Vice President Joe Biden and Mayor Pete Buttigieg of South Bend, Indiana, representing the more moderate wing.

Bloomberg, the chief executive officer and founder of Bloomberg LP, has been a leading advocate and philanthropist on the issues of climate change and gun violence. (QNA)